On Tue, Feb 20, 2024 at 02:09:18PM +0900, David Stevens wrote: > On Tue, Feb 20, 2024 at 1:06 PM Zhu, Lingshan <lingshan....@intel.com> wrote: > > On 2/19/2024 2:46 PM, David Stevens wrote: > > > On Sun, Feb 18, 2024 at 11:11 PM Michael S. Tsirkin <m...@redhat.com> > > > wrote: > > >> On Sun, Feb 18, 2024 at 09:23:06PM +0800, Zhu Lingshan wrote: > > >>> This commit allows the driver to suspend the device by > > >>> introducing a new status bit SUSPEND in device_status. > > >>> > > >>> This commit also introduce a new feature bit VIRTIO_F_SUSPEND > > >>> which indicating whether the device support SUSPEND. > > >>> > > >>> This SUSPEND bit is transport-independent. > > >>> > > >>> Signed-off-by: Zhu Lingshan <lingshan....@intel.com> > > >>> Signed-off-by: Jason Wang <jasow...@redhat.com> > > >>> Signed-off-by: Eugenio Pérez <epere...@redhat.com> > > >> Could we get some kind of dscription how this has taken into > > >> consideration the proposal from David Stevens? > > >> > > >> I find it really tiring when there are competing patches with authors > > >> ignoring each other's work and leaving it up to reviewers to > > >> figure out how do the patches compare. > > > This patch looks like it could be used to implement my use case. > > > However, parts of it are a bit vague and imprecise, so it's hard to > > > actually say whether my use case would actually be covered by a > > > specific implementation of this proposal. > > I am on vacation till this Friday. Shall we co-work on this and > > post something new together? > > That works for me. > > > > > > > To give specifics on what I'm trying to do, I need to allow a guest > > > with virtio-pci devices (including stateful devices like virtio-fs and > > > virtio-gpu) to be put into S1/S3, and to allow the virtio-pci devices > > > to wake up the guest (currently via an ACPI GPE, but eventually via a > > > native PCI PME). This is all done on a consumer device, so there is no > > > need for snapshotting or for live migration. > > Suspending is not dedicated only for live migration. > > For your use case, shall we add a new PCI section like saying: when entering > > SUSPEND, the device should get into S1/S3 and other PCI specific steps > > in PCI transport charter. > > > > That is because PCI is a transport layer of virtio, controlling virtio > > states by PCI sounds like a layer violation. > > I don't know if it's really necessary to mandate that in the spec. > Sure, most systems are probably going to want to put the virtio-pci > device into S1/S3 after suspending virtio operation. But nothing > really breaks or goes wrong if we don't do that.
What I feel is worth mentioning in virtio spec is that just setting the status bit does not reduce power by itself, standard pci control is required for that. > > > > > >>> --- > > >>> content.tex | 34 ++++++++++++++++++++++++++++++++-- > > >>> 1 file changed, 32 insertions(+), 2 deletions(-) > > >>> > > >>> diff --git a/content.tex b/content.tex > > >>> index 0a62dce..3d656b5 100644 > > >>> --- a/content.tex > > >>> +++ b/content.tex > > >>> @@ -49,6 +49,9 @@ \section{\field{Device Status} Field}\label{sec:Basic > > >>> Facilities of a Virtio Dev > > >>> > > >>> \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has > > >>> experienced > > >>> an error from which it can't recover. > > >>> + > > >>> +\item[SUSPEND (16)] When VIRTIO_F_SUSPEND is negotiated, indicates > > >>> that the > > >>> + device has been suspended by the driver. > > >>> \end{description} > > >>> > > >>> The \field{device status} field starts out as 0, and is reinitialized > > >>> to 0 by > > >>> @@ -73,6 +76,10 @@ \section{\field{Device Status} > > >>> Field}\label{sec:Basic Facilities of a Virtio Dev > > >>> recover by issuing a reset. > > >>> \end{note} > > >>> > > >>> +The driver SHOULD NOT set SUSPEND if FEATURES_OK is not set. > > >>> + > > >>> +When setting SUSPEND, the driver MUST re-read \field{device status} to > > >>> ensure the SUSPEND bit is set. > > >>> + > > >>> \devicenormative{\subsection}{Device Status Field}{Basic Facilities > > >>> of a Virtio Device / Device Status Field} > > >>> > > >>> The device MUST NOT consume buffers or send any used buffer > > >>> @@ -82,6 +89,25 @@ \section{\field{Device Status} > > >>> Field}\label{sec:Basic Facilities of a Virtio Dev > > >>> that a reset is needed. If DRIVER_OK is set, after it sets > > >>> DEVICE_NEEDS_RESET, the device > > >>> MUST send a device configuration change notification to the driver. > > >>> > > >>> +The device MUST ignore SUSPEND if FEATURES_OK is not set. > > >>> + > > >>> +The device MUST ignore SUSPEND if VIRTIO_F_SUSPEND is not negotiated. > > >>> + > > >>> +The device SHOULD allow settings to \field{device status} even when > > >>> SUSPEND is set. > > >>> + > > >>> +If VIRTIO_F_SUSPEND is negotiated and SUSPEND is set, the device > > >>> SHOULD clear SUSPEND > > >>> +and resumes operation upon DRIVER_OK. > > >>> + > > >>> +If VIRTIO_F_SUSPEND is negotiated, when the driver sets SUSPEND, > > >>> +the device SHOULD perform the following actions before presenting > > >>> SUSPEND bit in the \field{device status}: > > >>> + > > >>> +\begin{itemize} > > >>> +\item Stop consuming buffers of any virtqueues and mark all finished > > >>> descriptors as used. > > >>> +\item Wait until all descriptors that being processed to finish and > > >>> mark them as used. > > >>> +\item Flush all used buffer and send used buffer notifications to the > > >>> driver. > > > What's the difference between the previous three lines? They might be > > > trying to say different things, but there is a lot of overlap in how > > > they're phrased. That makes it hard to figure out exactly what they're > > > mandating. Is it something like: "stop processing new buffers", > > > "finish processing any buffers that are currently in flight", "mark > > > all finished buffers as used and send a used buffer notification"? > > sure, the "mark used" parts can merge. > > > > > > Also, what does this mean for devices where the driver places empty > > > buffers into the virtqueue that the device then holds on to for an > > > indeterminate period of time (e.g. network receiveq, balloon statsq, > > > etc)? > > The device doesn't process them. > > Oh, I should add: The driver should not make any more buffers available > > to the device. > > > > > >>> +\item Pause its operation except \field{device status} and preserve > > >>> configurations in its Device Configuration Space, see \ref{sec:Basic > > >>> Facilities of a Virtio Device / Device Configuration Space} > > > What does "Pause its operation except device status" mean? The word > > > "operation" is vague, what exactly does it include? Also what does > > > "operate its device status" mean? > > Because we need to resume the device after suspending it by setting > > DRIVER_OK, so we need the device status > > filed keel alive. > > This doesn't answer what "Pause its operation" means. If the > specification is not precise, then the best that can be implemented is > for a specific device to be compatible with a specific driver. If we > actually want portable, spec-compliant devices, we need explicitly > defined MUST/MUST NOT requirements. Presumably we want something like: > > A device MUST not send notifications while suspended. > A device MUST ignore writes to its device configuration while > suspended, except for writes to the device status byte. > A driver MUST NOT write to a suspended device's configuration space, > except for the device status byte. > > We also need to specify what a suspended device is allowed to do with > buffers it owns. For my use case of just suspending/resuming a VM, I > don't think it particularly matters if a suspended device writes to > buffers or descriptors while it is suspended. But based on what you > wrote about finishing any in-flight processing of buffers and not > processing any empty buffers, presumably you would prefer that the > device not be allowed to access buffer/descriptors while it is > suspended? That approach is probably cleaner from a pure specification > standpoint, but it's also more restrictive for devices and thus harder > to properly implement. > > -David Of course it's a bit harder. It does not sound too restrictive to me: as driver will not be adding new buffers, if device is still processing queues just defer setting the bit until it does not. --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org