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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: MWHPR11MB1886.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 18fa02a9-59a1-409d-0460-08d9264f0557 X-MS-Exchange-CrossTenant-originalarrivaltime: 03 Jun 2021 05:18:28.2217 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: wl0dMUtiQ7JDiFSYEC6Ib9IZStxGQ6DC/98ZSqXYERuKooXWXU9Jlubh+/589vwYRgbyeDgH2mxZgX6GkNw1WA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR11MB1471 X-OriginatorOrg: intel.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Alex Williamson > Sent: Thursday, June 3, 2021 12:15 PM >=20 > On Thu, 3 Jun 2021 03:22:27 +0000 > "Tian, Kevin" wrote: >=20 > > > From: Alex Williamson > > > Sent: Thursday, June 3, 2021 10:51 AM > > > > > > On Wed, 2 Jun 2021 19:45:36 -0300 > > > Jason Gunthorpe wrote: > > > > > > > On Wed, Jun 02, 2021 at 02:37:34PM -0600, Alex Williamson wrote: > > > > > > > > > Right. I don't follow where you're jumping to relaying DMA_PTE_S= NP > > > > > from the guest page table... what page table? > > > > > > > > I see my confusion now, the phrasing in your earlier remark led me > > > > think this was about allowing the no-snoop performance enhancement > in > > > > some restricted way. > > > > > > > > It is really about blocking no-snoop 100% of the time and then > > > > disabling the dangerous wbinvd when the block is successful. > > > > > > > > Didn't closely read the kvm code :\ > > > > > > > > If it was about allowing the optimization then I'd expect the guest= to > > > > enable no-snoopable regions via it's vIOMMU and realize them to the > > > > hypervisor and plumb the whole thing through. Hence my remark about > > > > the guest page tables.. > > > > > > > > So really the test is just 'were we able to block it' ? > > > > > > Yup. Do we really still consider that there's some performance benef= it > > > to be had by enabling a device to use no-snoop? This seems largely a > > > legacy thing. > > > > Yes, there is indeed performance benefit for device to use no-snoop, > > e.g. 8K display and some imaging processing path, etc. The problem is > > that the IOMMU for such devices is typically a different one from the > > default IOMMU for most devices. This special IOMMU may not have > > the ability of enforcing snoop on no-snoop PCI traffic then this fact > > must be understood by KVM to do proper mtrr/pat/wbinvd virtualization > > for such devices to work correctly. >=20 > The case where the IOMMU does not support snoop-control for such a > device already works fine, we can't prevent no-snoop so KVM will > emulate wbinvd. The harder one is if we should opt to allow no-snoop > even if the IOMMU does support snoop-control. In other discussion we are leaning toward a per-device capability reporting scheme through /dev/ioasid (or /dev/iommu as the new name). It seems natural to also allow setting a capability e.g. no- snoop for a device if underlying IOMMU driver allows it. >=20 > > > > > This support existed before mdev, IIRC we needed it for direct > > > > > assignment of NVIDIA GPUs. > > > > > > > > Probably because they ignored the disable no-snoop bits in the cont= rol > > > > block, or reset them in some insane way to "fix" broken bioses and > > > > kept using it even though by all rights qemu would have tried hard = to > > > > turn it off via the config space. Processing no-snoop without a > > > > working wbinvd would be fatal. Yeesh > > > > > > > > But Ok, back the /dev/ioasid. This answers a few lingering question= s I > > > > had.. > > > > > > > > 1) Mixing IOMMU_CAP_CACHE_COHERENCY > > > and !IOMMU_CAP_CACHE_COHERENCY > > > > domains. > > > > > > > > This doesn't actually matter. If you mix them together then kvm > > > > will turn on wbinvd anyhow, so we don't need to use the > DMA_PTE_SNP > > > > anywhere in this VM. > > > > > > > > This if two IOMMU's are joined together into a single /dev/ioasi= d > > > > then we can just make them both pretend to be > > > > !IOMMU_CAP_CACHE_COHERENCY and both not set IOMMU_CACHE. > > > > > > Yes and no. Yes, if any domain is !IOMMU_CAP_CACHE_COHERENCY > then > > > we > > > need to emulate wbinvd, but no we'll use IOMMU_CACHE any time it's > > > available based on the per domain support available. That gives us t= he > > > most consistent behavior, ie. we don't have VMs emulating wbinvd > > > because they used to have a device attached where the domain required > > > it and we can't atomically remap with new flags to perform the same a= s > > > a VM that never had that device attached in the first place. > > > > > > > 2) How to fit this part of kvm in some new /dev/ioasid world > > > > > > > > What we want to do here is iterate over every ioasid associated > > > > with the group fd that is passed into kvm. > > > > > > Yeah, we need some better names, binding a device to an ioasid (fd) b= ut > > > then attaching a device to an allocated ioasid (non-fd)... I assume > > > you're talking about the latter ioasid. > > > > > > > Today the group fd has a single container which specifies the > > > > single ioasid so this is being done trivially. > > > > > > > > To reorg we want to get the ioasid from the device not the > > > > group (see my note to David about the groups vs device rational) > > > > > > > > This is just iterating over each vfio_device in the group and > > > > querying the ioasid it is using. > > > > > > The IOMMU API group interfaces is largely iommu_group_for_each_dev() > > > anyway, we still need to account for all the RIDs and aliases of a > > > group. > > > > > > > Or perhaps more directly: an op attaching the vfio_device to the > > > > kvm and having some simple helper > > > > '(un)register ioasid with kvm (kvm, ioasid)' > > > > that the vfio_device driver can call that just sorts this out. > > > > > > We could almost eliminate the device notion altogether here, use an > > > ioasidfd_for_each_ioasid() but we really want a way to trigger on eac= h > > > change to the composition of the device set for the ioasid, which is > > > why we currently do it on addition or removal of a group, where the > > > group has a consistent set of IOMMU properties. Register a notifier > > > callback via the ioasidfd? Thanks, > > > > > > > When discussing I/O page fault support in another thread, the consensus > > is that an device handle will be registered (by user) or allocated (ret= urn > > to user) in /dev/ioasid when binding the device to ioasid fd. From this > > angle we can register {ioasid_fd, device_handle} to KVM and then call > > something like ioasidfd_device_is_coherent() to get the property. > > Anyway the coherency is a per-device property which is not changed > > by how many I/O page tables are attached to it. >=20 > The mechanics are different, but this is pretty similar in concept to > KVM learning coherence using the groupfd today. Do we want to > compromise on kernel control of wbinvd emulation to allow userspace to > make such decisions? Ownership of a device might be reason enough to > allow the user that privilege. Thanks, >=20 I think so. In the end it's still decided by the underlying IOMMU driver.=20 If the IOMMU driver doesn't allow user to opt for no-snoop, it's exactly=20 same as today's groupfd approach. Otherwise an user-opted policy=20 implies that the decision is delegated to userspace.=20 Thanks Kevin