Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
On 2022-03-22 11:41, Mika Westerberg wrote:
Hi Robin,
I tried this now on two Intel systems. One with integrated Thunderbolt
and one with discrete. There was a small issue, see below but once fixed
it worked as expected :)
On Fri, Mar 18, 2022 at 05:42:58PM +, Robin Murphy wrote:
Between me trying to get rid of iommu_present() and Mario wanting to
support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has shown
that the iommu_dma_protection attribute is being far too optimistic.
Even if an IOMMU might be present for some PCI segment in the system,
that doesn't necessarily mean it provides translation for the device(s)
we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does
is tell us that memory was protected before the kernel was loaded, and
prevent the user from disabling the intel-iommu driver entirely. While
that lets us assume kernel integrity, what matters for actual runtime
DMA protection is whether we trust individual devices, based on the
"external facing" property that we expect firmware to describe for
Thunderbolt ports.
It's proven challenging to determine the appropriate ports accurately
given the variety of possible topologies, so while still not getting a
perfect answer, by putting enough faith in firmware we can at least get
a good bit closer. If we can see that any device near a Thunderbolt NHI
has all the requisites for Kernel DMA Protection, chances are that it
*is* a relevant port, but moreover that implies that firmware is playing
the game overall, so we'll use that to assume that all Thunderbolt ports
should be correctly marked and thus will end up fully protected.
CC: Mario Limonciello
Signed-off-by: Robin Murphy
---
v2: Give up trying to look for specific devices, just look for evidence
that firmware cares at all.
drivers/thunderbolt/domain.c | 12 +++
drivers/thunderbolt/nhi.c| 41
include/linux/thunderbolt.h | 2 ++
3 files changed, 46 insertions(+), 9 deletions(-)
diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
index 7018d959f775..2889a214dadc 100644
--- a/drivers/thunderbolt/domain.c
+++ b/drivers/thunderbolt/domain.c
@@ -7,9 +7,7 @@
*/
#include
-#include
#include
-#include
#include
#include
#include
@@ -257,13 +255,9 @@ static ssize_t iommu_dma_protection_show(struct device
*dev,
struct device_attribute *attr,
char *buf)
{
- /*
-* Kernel DMA protection is a feature where Thunderbolt security is
-* handled natively using IOMMU. It is enabled when IOMMU is
-* enabled and ACPI DMAR table has DMAR_PLATFORM_OPT_IN set.
-*/
- return sprintf(buf, "%d\n",
- iommu_present(_bus_type) && dmar_platform_optin());
+ struct tb *tb = container_of(dev, struct tb, dev);
+
+ return sysfs_emit(buf, "%d\n", tb->nhi->iommu_dma_protection);
}
static DEVICE_ATTR_RO(iommu_dma_protection);
diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
index c73da0532be4..9e396e283792 100644
--- a/drivers/thunderbolt/nhi.c
+++ b/drivers/thunderbolt/nhi.c
@@ -14,6 +14,7 @@
#include
#include
#include
+#include
#include
#include
#include
@@ -1102,6 +1103,45 @@ static void nhi_check_quirks(struct tb_nhi *nhi)
nhi->quirks |= QUIRK_AUTO_CLEAR_INT;
}
+static int nhi_check_iommu_pdev(struct pci_dev *pdev, void *data)
+{
+ if (!pdev->untrusted ||
+ !dev_iommu_capable(>dev, IOMMU_CAP_PRE_BOOT_PROTECTION))
This one needs to take the pdev->external_facing into account too
because most of the time there are no existing tunnels when the driver
is loaded so we only see the PCIe root/downstream port. I think this is
enough actually:
if (!pdev->external_facing ||
!dev_iommu_capable(>dev, IOMMU_CAP_PRE_BOOT_PROTECTION))
Ah yes, my bad, for some reason I got the misapprehension into my head
that untrusted was propagated to the port as well, not just the devices
behind it. I'll fix this and tweak the comment below to match.
+ return 0;
+ *(bool *)data = true;
+ return 1; /* Stop walking */
+}
+
+static void nhi_check_iommu(struct tb_nhi *nhi)
+{
+ struct pci_bus *bus = nhi->pdev->bus;
+ bool port_ok = false;
+
+ /*
+* Ideally what we'd do here is grab every PCI device that
+* represents a tunnelling adapter for this NHI and check their
+* status directly, but unfortunately USB4 seems to make it
+* obnoxiously difficult to reliably make any correlation.
+*
+* So for now we'll have to bodge it... Hoping that the system
+* is at least sane enough that an adapter is in the same PCI
+* segment as its NHI, if we can find *something* on that segment
+* which meets the requirements for Kernel DMA Protection, we'll
+
Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
Hi Robin,
I tried this now on two Intel systems. One with integrated Thunderbolt
and one with discrete. There was a small issue, see below but once fixed
it worked as expected :)
On Fri, Mar 18, 2022 at 05:42:58PM +, Robin Murphy wrote:
> Between me trying to get rid of iommu_present() and Mario wanting to
> support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has shown
> that the iommu_dma_protection attribute is being far too optimistic.
> Even if an IOMMU might be present for some PCI segment in the system,
> that doesn't necessarily mean it provides translation for the device(s)
> we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does
> is tell us that memory was protected before the kernel was loaded, and
> prevent the user from disabling the intel-iommu driver entirely. While
> that lets us assume kernel integrity, what matters for actual runtime
> DMA protection is whether we trust individual devices, based on the
> "external facing" property that we expect firmware to describe for
> Thunderbolt ports.
>
> It's proven challenging to determine the appropriate ports accurately
> given the variety of possible topologies, so while still not getting a
> perfect answer, by putting enough faith in firmware we can at least get
> a good bit closer. If we can see that any device near a Thunderbolt NHI
> has all the requisites for Kernel DMA Protection, chances are that it
> *is* a relevant port, but moreover that implies that firmware is playing
> the game overall, so we'll use that to assume that all Thunderbolt ports
> should be correctly marked and thus will end up fully protected.
>
> CC: Mario Limonciello
> Signed-off-by: Robin Murphy
> ---
>
> v2: Give up trying to look for specific devices, just look for evidence
> that firmware cares at all.
>
> drivers/thunderbolt/domain.c | 12 +++
> drivers/thunderbolt/nhi.c| 41
> include/linux/thunderbolt.h | 2 ++
> 3 files changed, 46 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
> index 7018d959f775..2889a214dadc 100644
> --- a/drivers/thunderbolt/domain.c
> +++ b/drivers/thunderbolt/domain.c
> @@ -7,9 +7,7 @@
> */
>
> #include
> -#include
> #include
> -#include
> #include
> #include
> #include
> @@ -257,13 +255,9 @@ static ssize_t iommu_dma_protection_show(struct device
> *dev,
>struct device_attribute *attr,
>char *buf)
> {
> - /*
> - * Kernel DMA protection is a feature where Thunderbolt security is
> - * handled natively using IOMMU. It is enabled when IOMMU is
> - * enabled and ACPI DMAR table has DMAR_PLATFORM_OPT_IN set.
> - */
> - return sprintf(buf, "%d\n",
> -iommu_present(_bus_type) && dmar_platform_optin());
> + struct tb *tb = container_of(dev, struct tb, dev);
> +
> + return sysfs_emit(buf, "%d\n", tb->nhi->iommu_dma_protection);
> }
> static DEVICE_ATTR_RO(iommu_dma_protection);
>
> diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
> index c73da0532be4..9e396e283792 100644
> --- a/drivers/thunderbolt/nhi.c
> +++ b/drivers/thunderbolt/nhi.c
> @@ -14,6 +14,7 @@
> #include
> #include
> #include
> +#include
> #include
> #include
> #include
> @@ -1102,6 +1103,45 @@ static void nhi_check_quirks(struct tb_nhi *nhi)
> nhi->quirks |= QUIRK_AUTO_CLEAR_INT;
> }
>
> +static int nhi_check_iommu_pdev(struct pci_dev *pdev, void *data)
> +{
> + if (!pdev->untrusted ||
> + !dev_iommu_capable(>dev, IOMMU_CAP_PRE_BOOT_PROTECTION))
This one needs to take the pdev->external_facing into account too
because most of the time there are no existing tunnels when the driver
is loaded so we only see the PCIe root/downstream port. I think this is
enough actually:
if (!pdev->external_facing ||
!dev_iommu_capable(>dev, IOMMU_CAP_PRE_BOOT_PROTECTION))
> + return 0;
> + *(bool *)data = true;
> + return 1; /* Stop walking */
> +}
> +
> +static void nhi_check_iommu(struct tb_nhi *nhi)
> +{
> + struct pci_bus *bus = nhi->pdev->bus;
> + bool port_ok = false;
> +
> + /*
> + * Ideally what we'd do here is grab every PCI device that
> + * represents a tunnelling adapter for this NHI and check their
> + * status directly, but unfortunately USB4 seems to make it
> + * obnoxiously difficult to reliably make any correlation.
> + *
> + * So for now we'll have to bodge it... Hoping that the system
> + * is at least sane enough that an adapter is in the same PCI
> + * segment as its NHI, if we can find *something* on that segment
> + * which meets the requirements for Kernel DMA Protection, we'll
> + * take that to imply that firmware is aware and has (hopefully)
> + * done the right thing in general. We need to know
Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
On Fri, Mar 18, 2022 at 05:42:58PM +, Robin Murphy wrote: > Between me trying to get rid of iommu_present() and Mario wanting to > support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has shown > that the iommu_dma_protection attribute is being far too optimistic. > Even if an IOMMU might be present for some PCI segment in the system, > that doesn't necessarily mean it provides translation for the device(s) > we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does > is tell us that memory was protected before the kernel was loaded, and > prevent the user from disabling the intel-iommu driver entirely. While > that lets us assume kernel integrity, what matters for actual runtime > DMA protection is whether we trust individual devices, based on the > "external facing" property that we expect firmware to describe for > Thunderbolt ports. > > It's proven challenging to determine the appropriate ports accurately > given the variety of possible topologies, so while still not getting a > perfect answer, by putting enough faith in firmware we can at least get > a good bit closer. If we can see that any device near a Thunderbolt NHI > has all the requisites for Kernel DMA Protection, chances are that it > *is* a relevant port, but moreover that implies that firmware is playing > the game overall, so we'll use that to assume that all Thunderbolt ports > should be correctly marked and thus will end up fully protected. > > CC: Mario Limonciello > Signed-off-by: Robin Murphy Looks sensible to me: Acked-by: Christoph Hellwig ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
RE: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
[Public] > -Original Message- > From: Robin Murphy > Sent: Monday, March 21, 2022 06:12 > To: mika.westerb...@linux.intel.com; Limonciello, Mario > > Cc: j...@8bytes.org; baolu...@linux.intel.com; andreas.noe...@gmail.com; > michael.ja...@intel.com; yehezkel...@gmail.com; iommu@lists.linux- > foundation.org; linux-...@vger.kernel.org; linux-ker...@vger.kernel.org; > h...@lst.de > Subject: Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection > more accurate > > On 2022-03-21 10:58, mika.westerb...@linux.intel.com wrote: > > Hi Mario, > > > > On Fri, Mar 18, 2022 at 10:29:59PM +, Limonciello, Mario wrote: > >> [Public] > >> > >>> Between me trying to get rid of iommu_present() and Mario wanting to > >>> support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has > >>> shown > >>> that the iommu_dma_protection attribute is being far too optimistic. > >>> Even if an IOMMU might be present for some PCI segment in the > system, > >>> that doesn't necessarily mean it provides translation for the device(s) > >>> we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really > does > >>> is tell us that memory was protected before the kernel was loaded, and > >>> prevent the user from disabling the intel-iommu driver entirely. While > >>> that lets us assume kernel integrity, what matters for actual runtime > >>> DMA protection is whether we trust individual devices, based on the > >>> "external facing" property that we expect firmware to describe for > >>> Thunderbolt ports. > >>> > >>> It's proven challenging to determine the appropriate ports accurately > >>> given the variety of possible topologies, so while still not getting a > >>> perfect answer, by putting enough faith in firmware we can at least get > >>> a good bit closer. If we can see that any device near a Thunderbolt NHI > >>> has all the requisites for Kernel DMA Protection, chances are that it > >>> *is* a relevant port, but moreover that implies that firmware is playing > >>> the game overall, so we'll use that to assume that all Thunderbolt ports > >>> should be correctly marked and thus will end up fully protected. > >>> > >> > >> This approach looks generally good to me. I do worry a little bit about > older > >> systems that didn't set ExternalFacingPort in the FW but were previously > setting > >> iommu_dma_protection, but I think that those could be treated on a > quirk > >> basis to set PCI IDs for those root ports as external facing if/when they > come > >> up. > > > > There are no such systems out there AFAICT. > > And even if there are, as above they've never actually been fully > protected and still won't be, so it's arguably a good thing for them to > stop thinking so. I was meaning that if this situation comes up that we could tag the PCI IDs for those root ports as ExternalFacing in drivers/pci/quirks.c so that the protection "is" enacted for them even if it was missing from the firmware. > > >> I'll send up a follow up patch that adds the AMD ACPI table check. > >> If it looks good can roll it into your series for v3, or if this series > >> goes > >> as is for v2 it can come on its own. > >> > >>> CC: Mario Limonciello > >>> Signed-off-by: Robin Murphy > >>> --- > >>> > >>> v2: Give up trying to look for specific devices, just look for evidence > >>> that firmware cares at all. > >> > >> I still do think you could know exactly which devices to use if you're in > >> SW CM mode, but I guess the consensus is to not bifurcate the way this > >> can be checked. > > > > Indeed. > > > > The patch looks good to me now. I will give it a try on a couple of > > systems later today or tomorrow and let you guys know how it went. I > > don't expect any problems but let's see. > > > > Thanks a lot Robin for working on this :) > > Heh, let's just hope the other half-dozen or so subsystems I need to > touch for this IOMMU cleanup aren't all quite as involved as this turned > out to be :) Thanks a lot for this effort! > > Cheers, > Robin. ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
On 2022-03-21 10:58, mika.westerb...@linux.intel.com wrote: Hi Mario, On Fri, Mar 18, 2022 at 10:29:59PM +, Limonciello, Mario wrote: [Public] Between me trying to get rid of iommu_present() and Mario wanting to support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has shown that the iommu_dma_protection attribute is being far too optimistic. Even if an IOMMU might be present for some PCI segment in the system, that doesn't necessarily mean it provides translation for the device(s) we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does is tell us that memory was protected before the kernel was loaded, and prevent the user from disabling the intel-iommu driver entirely. While that lets us assume kernel integrity, what matters for actual runtime DMA protection is whether we trust individual devices, based on the "external facing" property that we expect firmware to describe for Thunderbolt ports. It's proven challenging to determine the appropriate ports accurately given the variety of possible topologies, so while still not getting a perfect answer, by putting enough faith in firmware we can at least get a good bit closer. If we can see that any device near a Thunderbolt NHI has all the requisites for Kernel DMA Protection, chances are that it *is* a relevant port, but moreover that implies that firmware is playing the game overall, so we'll use that to assume that all Thunderbolt ports should be correctly marked and thus will end up fully protected. This approach looks generally good to me. I do worry a little bit about older systems that didn't set ExternalFacingPort in the FW but were previously setting iommu_dma_protection, but I think that those could be treated on a quirk basis to set PCI IDs for those root ports as external facing if/when they come up. There are no such systems out there AFAICT. And even if there are, as above they've never actually been fully protected and still won't be, so it's arguably a good thing for them to stop thinking so. I'll send up a follow up patch that adds the AMD ACPI table check. If it looks good can roll it into your series for v3, or if this series goes as is for v2 it can come on its own. CC: Mario Limonciello Signed-off-by: Robin Murphy --- v2: Give up trying to look for specific devices, just look for evidence that firmware cares at all. I still do think you could know exactly which devices to use if you're in SW CM mode, but I guess the consensus is to not bifurcate the way this can be checked. Indeed. The patch looks good to me now. I will give it a try on a couple of systems later today or tomorrow and let you guys know how it went. I don't expect any problems but let's see. Thanks a lot Robin for working on this :) Heh, let's just hope the other half-dozen or so subsystems I need to touch for this IOMMU cleanup aren't all quite as involved as this turned out to be :) Cheers, Robin. ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
Re: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
Hi Mario, On Fri, Mar 18, 2022 at 10:29:59PM +, Limonciello, Mario wrote: > [Public] > > > Between me trying to get rid of iommu_present() and Mario wanting to > > support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has > > shown > > that the iommu_dma_protection attribute is being far too optimistic. > > Even if an IOMMU might be present for some PCI segment in the system, > > that doesn't necessarily mean it provides translation for the device(s) > > we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does > > is tell us that memory was protected before the kernel was loaded, and > > prevent the user from disabling the intel-iommu driver entirely. While > > that lets us assume kernel integrity, what matters for actual runtime > > DMA protection is whether we trust individual devices, based on the > > "external facing" property that we expect firmware to describe for > > Thunderbolt ports. > > > > It's proven challenging to determine the appropriate ports accurately > > given the variety of possible topologies, so while still not getting a > > perfect answer, by putting enough faith in firmware we can at least get > > a good bit closer. If we can see that any device near a Thunderbolt NHI > > has all the requisites for Kernel DMA Protection, chances are that it > > *is* a relevant port, but moreover that implies that firmware is playing > > the game overall, so we'll use that to assume that all Thunderbolt ports > > should be correctly marked and thus will end up fully protected. > > > > This approach looks generally good to me. I do worry a little bit about older > systems that didn't set ExternalFacingPort in the FW but were previously > setting > iommu_dma_protection, but I think that those could be treated on a quirk > basis to set PCI IDs for those root ports as external facing if/when they come > up. There are no such systems out there AFAICT. > I'll send up a follow up patch that adds the AMD ACPI table check. > If it looks good can roll it into your series for v3, or if this series goes > as is for v2 it can come on its own. > > > CC: Mario Limonciello > > Signed-off-by: Robin Murphy > > --- > > > > v2: Give up trying to look for specific devices, just look for evidence > > that firmware cares at all. > > I still do think you could know exactly which devices to use if you're in > SW CM mode, but I guess the consensus is to not bifurcate the way this > can be checked. Indeed. The patch looks good to me now. I will give it a try on a couple of systems later today or tomorrow and let you guys know how it went. I don't expect any problems but let's see. Thanks a lot Robin for working on this :) ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
RE: [PATCH v2 2/2] thunderbolt: Make iommu_dma_protection more accurate
[Public]
> Between me trying to get rid of iommu_present() and Mario wanting to
> support the AMD equivalent of DMAR_PLATFORM_OPT_IN, scrutiny has
> shown
> that the iommu_dma_protection attribute is being far too optimistic.
> Even if an IOMMU might be present for some PCI segment in the system,
> that doesn't necessarily mean it provides translation for the device(s)
> we care about. Furthermore, all that DMAR_PLATFORM_OPT_IN really does
> is tell us that memory was protected before the kernel was loaded, and
> prevent the user from disabling the intel-iommu driver entirely. While
> that lets us assume kernel integrity, what matters for actual runtime
> DMA protection is whether we trust individual devices, based on the
> "external facing" property that we expect firmware to describe for
> Thunderbolt ports.
>
> It's proven challenging to determine the appropriate ports accurately
> given the variety of possible topologies, so while still not getting a
> perfect answer, by putting enough faith in firmware we can at least get
> a good bit closer. If we can see that any device near a Thunderbolt NHI
> has all the requisites for Kernel DMA Protection, chances are that it
> *is* a relevant port, but moreover that implies that firmware is playing
> the game overall, so we'll use that to assume that all Thunderbolt ports
> should be correctly marked and thus will end up fully protected.
>
This approach looks generally good to me. I do worry a little bit about older
systems that didn't set ExternalFacingPort in the FW but were previously setting
iommu_dma_protection, but I think that those could be treated on a quirk
basis to set PCI IDs for those root ports as external facing if/when they come
up.
I'll send up a follow up patch that adds the AMD ACPI table check.
If it looks good can roll it into your series for v3, or if this series goes
as is for v2 it can come on its own.
> CC: Mario Limonciello
> Signed-off-by: Robin Murphy
> ---
>
> v2: Give up trying to look for specific devices, just look for evidence
> that firmware cares at all.
I still do think you could know exactly which devices to use if you're in
SW CM mode, but I guess the consensus is to not bifurcate the way this
can be checked.
>
> drivers/thunderbolt/domain.c | 12 +++
> drivers/thunderbolt/nhi.c| 41
>
> include/linux/thunderbolt.h | 2 ++
> 3 files changed, 46 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
> index 7018d959f775..2889a214dadc 100644
> --- a/drivers/thunderbolt/domain.c
> +++ b/drivers/thunderbolt/domain.c
> @@ -7,9 +7,7 @@
> */
>
> #include
> -#include
> #include
> -#include
> #include
> #include
> #include
> @@ -257,13 +255,9 @@ static ssize_t iommu_dma_protection_show(struct
> device *dev,
>struct device_attribute *attr,
>char *buf)
> {
> - /*
> - * Kernel DMA protection is a feature where Thunderbolt security is
> - * handled natively using IOMMU. It is enabled when IOMMU is
> - * enabled and ACPI DMAR table has DMAR_PLATFORM_OPT_IN set.
> - */
> - return sprintf(buf, "%d\n",
> -iommu_present(_bus_type) &&
> dmar_platform_optin());
> + struct tb *tb = container_of(dev, struct tb, dev);
> +
> + return sysfs_emit(buf, "%d\n", tb->nhi->iommu_dma_protection);
> }
> static DEVICE_ATTR_RO(iommu_dma_protection);
>
> diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
> index c73da0532be4..9e396e283792 100644
> --- a/drivers/thunderbolt/nhi.c
> +++ b/drivers/thunderbolt/nhi.c
> @@ -14,6 +14,7 @@
> #include
> #include
> #include
> +#include
> #include
> #include
> #include
> @@ -1102,6 +1103,45 @@ static void nhi_check_quirks(struct tb_nhi *nhi)
> nhi->quirks |= QUIRK_AUTO_CLEAR_INT;
> }
>
> +static int nhi_check_iommu_pdev(struct pci_dev *pdev, void *data)
> +{
> + if (!pdev->untrusted ||
> + !dev_iommu_capable(>dev,
> IOMMU_CAP_PRE_BOOT_PROTECTION))
> + return 0;
> + *(bool *)data = true;
> + return 1; /* Stop walking */
> +}
> +
> +static void nhi_check_iommu(struct tb_nhi *nhi)
> +{
> + struct pci_bus *bus = nhi->pdev->bus;
> + bool port_ok = false;
> +
> + /*
> + * Ideally what we'd do here is grab every PCI device that
> + * represents a tunnelling adapter for this NHI and check their
> + * status directly, but unfortunately USB4 seems to make it
> + * obnoxiously difficult to reliably make any correlation.
> + *
> + * So for now we'll have to bodge it... Hoping that the system
> + * is at least sane enough that an adapter is in the same PCI
> + * segment as its NHI, if we can find *something* on that segment
> + * which meets the requirements for Kernel DMA Protection, we'll
> + * take that to imply that
