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

[Tina Zhang]

Hi,

On 8/11/23 00:27, Jason Gunthorpe wrote:
> On Thu, Aug 10, 2023 at 07:49:11AM +0000, Tian, Kevin wrote:
> > > From: Zhang, Tina <tina.zhang@xxxxxxxxx>
> > > Sent: Thursday, August 10, 2023 9:32 AM
> > >
> > > Hi,
> > >
> > > On 8/9/23 17:41, Tian, Kevin wrote:
> > > > > From: Zhang, Tina <tina.zhang@xxxxxxxxx>
> > > > > Sent: Tuesday, August 8, 2023 3:50 PM
> > > > >
> > > > > A sva domain's lifetime begins with binding a device to a mm and ends
> > > > > by releasing all the bound devices from that sva domain. Technically,
> > > > > there could be more than one sva domain identified by the mm PASID for
> > > > > the use of bound devices issuing DMA transactions.
> > > >
> > > > Could you elaborate it with some concrete examples which motivate
> > > > this change?
> > > The motivation is to remove the superfluous IOTLB invalidation in
> > > current VT-d driver.
> > >
> > > Currently, in VT-d driver, due to lacking shared sva domain info, in
> > > intel_flush_svm_range(), both iotlb and dev-tlb invalidation operations
> > > are performed per-device. However, difference devices could be behind
> > > one IOMMU (e.g., four devices are behind one IOMMU) and invoking iotlb
> > > per-device gives us more iotlb invalidation than necessary (4 iotlb
> > > invalidation instead of 1). This issue may give more performance impact
> > > when in a virtual machine guest, as currently we have one virtual VT-d
> > > for in front of those virtual devices.
> > >
> > >
> > > This patch fixes this issue by attaching shared sva domain information
> > > to mm, so that it can be utilized in the mm_notifier_ops callbacks.
> >
> > that is one of the motivations. e.g. another one as Jason suggested
> > is to cleanup to decouple the common sva logic from enqcmd. Both
> > should be mentioned in next version cover letter.
>
> I also want to purge all the de-duplication and refcounting code
> around mm's and sva_binds from the drivers. Eg see the mess this makes
> of SMMUv3.
>
> Core code provides a single iommu_domain per-mm for SVA. Driver can
> rely on this optimization and does not need to de-duplicate.
>
> Single domain tracks all attachments. Driver can optimize using that
> information by de-duplicating (eg ASID invalidation vs ATC
> invalidation)
>
> After this we need to fix the domain allocation op to add a
> 'alloc_domain_sva(dev, mm_struct)' op so that the drivers can setup
> their SVA domains fully in a nice lock-safe environment.
Agree. These can be added to the to-do list.

Regards,
-Tina
> Jason