On Tue, 26 May 2020 11:06:48 -0700 "Raj, Ashok" <ashok....@intel.com> wrote:
> Hi Alex, > > I was able to find better language in the IOMMU spec that gaurantees > the behavior we need. See below. > > > On Tue, May 05, 2020 at 09:34:14AM -0600, Alex Williamson wrote: > > On Tue, 5 May 2020 07:56:06 -0700 > > "Raj, Ashok" <ashok....@intel.com> wrote: > > > > > On Tue, May 05, 2020 at 08:05:14AM -0600, Alex Williamson wrote: > > > > On Mon, 4 May 2020 23:11:07 -0700 > > > > "Raj, Ashok" <ashok....@intel.com> wrote: > > > > > > > > > Hi Alex > > > > > > > > > > + Joerg, accidently missed in the Cc. > > > > > > > > > > On Mon, May 04, 2020 at 11:19:36PM -0600, Alex Williamson wrote: > > > > > > On Mon, 4 May 2020 21:42:16 -0700 > > > > > > Ashok Raj <ashok....@intel.com> wrote: > > > > > > > > > > > > > PCIe Spec recommends we can relax ACS requirement for RCIEP > > > > > > > devices. > > > > > > > > > > > > > > PCIe 5.0 Specification. > > > > > > > 6.12 Access Control Services (ACS) > > > > > > > Implementation of ACS in RCiEPs is permitted but not required. It > > > > > > > is > > > > > > > explicitly permitted that, within a single Root Complex, some > > > > > > > RCiEPs > > > > > > > implement ACS and some do not. It is strongly recommended that > > > > > > > Root Complex > > > > > > > implementations ensure that all accesses originating from RCiEPs > > > > > > > (PFs and VFs) without ACS capability are first subjected to > > > > > > > processing by > > > > > > > the Translation Agent (TA) in the Root Complex before further > > > > > > > decoding and > > > > > > > processing. The details of such Root Complex handling are outside > > > > > > > the scope > > > > > > > of this specification. > > > > > > > > > > > > > > > > > > > Is the language here really strong enough to make this change? ACS > > > > > > is > > > > > > an optional feature, so being permitted but not required is rather > > > > > > meaningless. The spec is also specifically avoiding the words > > > > > > "must" > > > > > > or "shall" and even when emphasized with "strongly", we still only > > > > > > have > > > > > > a recommendation that may or may not be honored. This seems like a > > > > > > weak basis for assuming that RCiEPs universally honor this > > > > > > recommendation. Thanks, > > > > > > > > > > > > > > > > We are speaking about PCIe spec, where people write it about 5 years > > > > > ahead > > > > > and every vendor tries to massage their product behavior with vague > > > > > words like this.. :) > > > > > > > > > > But honestly for any any RCiEP, or even integrated endpoints, there > > > > > is no way to send them except up north. These aren't behind a RP. > > > > > > > > But they are multi-function devices and the spec doesn't define routing > > > > within multifunction packages. A single function RCiEP will already be > > > > assumed isolated within its own group. > > > > > > That's right. The other two devices only have legacy PCI headers. So > > > they can't claim to be RCiEP's but just integrated endpoints. The legacy > > > devices don't even have a PCIe header. > > > > > > I honestly don't know why these are groped as MFD's in the first place. > > > > > > > > > > > > I did check with couple folks who are part of the SIG, and seem to > > > > > agree > > > > > that ACS treatment for RCiEP's doesn't mean much. > > > > > > > > > > I understand the language isn't strong, but it doesn't seem like ACS > > > > > should > > > > > be a strong requirement for RCiEP's and reasonable to relax. > > > > > > > > > > What are your thoughts? > > > > > > > > I think hardware vendors have ACS at their disposal to clarify when > > > > isolation is provided, otherwise vendors can submit quirks, but I don't > > > > see that the "strongly recommended" phrasing is sufficient to assume > > > > isolation between multifunction RCiEPs. Thanks, > > > > > > You point is that integrated MFD endpoints, without ACS, there is no > > > gaurantee to SW that they are isolated. > > > > > > As far as a quirk, do you think: > > > - a cmdline optput for integrated endpoints, and RCiEP's suffice? > > > along with a compile time default that is strict enforcement > > > - typical vid/did type exception list? > > > > > > A more generic way to ask for exception would be scalable until we can > > > stop > > > those type of integrated devices. Or we need to maintain these device > > > lists > > > for eternity. > > > > I don't think the language in the spec is anything sufficient to handle > > RCiEP uniquely. We've previously rejected kernel command line opt-outs > > for ACS, and the extent to which those patches still float around the > > user community and are blindly used to separate IOMMU groups are a > > testament to the failure of this approach. Users do not have a basis > > for enabling this sort of opt-out. The benefit is obvious in the IOMMU > > grouping, but the risk is entirely unknown. A kconfig option is even > > worse as that means if you consume a downstream kernel, the downstream > > maintainers might have decided universally that isolation is less > > important than functionality. > > We discussed this internally, and Intel vt-d spec does spell out clearly > in Section 3.16 Root-Complex Peer to Peer Considerations. The spec clearly > calls out that all p2p must be done on translated addresses and therefore > must go through the IOMMU. > > I suppose they should also have some similar platform gauranteed behavior > for RCiEP's or MFD's *Must* behave as follows. The language is strict and > when IOMMU is enabled in the platform, everything is sent up north to the > IOMMU agent. > > 3.16 Root-Complex Peer to Peer Considerations > When DMA remapping is enabled, peer-to-peer requests through the > Root-Complex must be handled > as follows: > • The input address in the request is translated (through first-level, > second-level or nested translation) to a host physical address (HPA). > The address decoding for peer addresses must be done only on the > translated HPA. Hardware implementations are free to further limit > peer-to-peer accesses to specific host physical address regions > (or to completely disallow peer-forwarding of translated requests). > • Since address translation changes the contents (address field) of the PCI > Express Transaction Layer Packet (TLP), for PCI Express peer-to-peer > requests with ECRC, the Root-Complex hardware must use the new ECRC > (re-computed with the translated address) if it decides to forward > the TLP as a peer request. > • Root-ports, and multi-function root-complex integrated endpoints, may > support additional peerto-peer control features by supporting PCI Express > Access Control Services (ACS) capability. Refer to ACS capability in > PCI Express specifications for details. That sounds like it might be a reasonable basis for quirking all RCiEPs on VT-d platforms if Intel is willing to stand behind it. Thanks, Alex
