Re: [PATCH v8 0/5] Share sva domains with all devices bound to a mm

[Baolu Lu]

On 10/18/23 12:41 AM, Jason Gunthorpe wrote:
> On Tue, Oct 17, 2023 at 08:47:57AM +0800, Tina Zhang wrote:
> > This series is to share sva(shared virtual addressing) domains with all
> > devices bound to one mm.
> >
> > Problem
> > -------
> > In the current iommu core code, sva domain is allocated per IOMMU group,
> > when device driver is binding a process address space to a device (which is
> > handled in iommu_sva_bind_device()). If one than more device is bound to
> > the same process address space, there must be more than one sva domain
> > instance, with each device having one. In other words, the sva domain
> > doesn't share between those devices bound to the same process address
> > space, and that leads to two problems:
> > 1) device driver has to duplicate sva domains with enqcmd, as those sva
> > domains have the same PASID and are relevant to one virtual address space.
> > This makes the sva domain handling complex in device drivers.
> > 2) IOMMU driver cannot get sufficient info of the IOMMUs that have
> > devices behind them bound to the same virtual address space, when handling
> > mmu_notifier_ops callbacks. As a result, IOMMU IOTLB invalidation is
> > performed per device instead of per IOMMU, and that may lead to
> > superfluous IOTLB invalidation issue, especially in a virtualization
> > environment where all devices may be behind one virtual IOMMU.
> >
> > Solution
> > --------
> > This patch-set tries to fix those two problems by allowing sharing sva
> > domains with all devices bound to a mm. To achieve this, a new structure
> > pointer is introduced to mm to replace the old PASID field, which can keep
> > the info of PASID as well as the corresponding shared sva domains.
> > Besides, function iommu_sva_bind_device() is updated to ensure a new sva
> > domain can only be allocated when the old ones cannot work for the IOMMU.
> > With these changes, a device driver can expect one sva domain could work
> > for per PASID instance(e.g., enqcmd PASID instance), and therefore may get
> > rid of handling sva domain duplication. Besides, IOMMU driver (e.g., intel
> > vt-d driver) can get sufficient info (e.g., the info of the IOMMUs having
> > their devices bound to one virtual address space) when handling
> > mmu_notifier_ops callbacks, to remove the redundant IOTLB invalidations.
> >
> > Arguably there shouldn't be more than one sva_domain with the same PASID,
> > and in any sane configuration there should be only 1 type of IOMMU driver
> > that needs only 1 SVA domain. However, in reality, IOMMUs on one platform
> > may not be identical to each other. Thus, attaching a sva domain that has
> > been successfully bound to device A behind a IOMMU A, to device B behind
> > IOMMU B may get failed due to the difference between IOMMU A and IOMMU
> > B. In this case, a new sva domain with the same PASID needs to be
> > allocated to work with IOMMU B. That's why we need a list to keep sva
> > domains of one PASID. For the platform where IOMMUs are compatible to each
> > other, there should be one sva domain in the list.
> >
> > v8:
> >   - CC more people
> >   - CCiommu@xxxxxxxxxxxxxxx  mailing list.
> >     When sending version 7, some issue happened in my CC list and that caused
> >     version 7 wasn't sent toiommu@xxxxxxxxxxxxxxx.
> >   - Rebase to v6.6-rc6 and make a few format changes.
> You should based it on Joerg's tree so he can take it without
> conflcits.
>
> The conflicts are trivial though (Take Michael's version and switch
> mm->pasid with mm_get_enqcmd_pasid(mm))
>
> It looks fine, please lets get it in this cycle, the ARM and AMD SVA
> series depend on it.

The vt-d driver also has series depending on it.

https://lore.kernel.org/linux-iommu/20231017032045.114868-1-tina.zhang@xxxxxxxxx/

Best regards,
baolu