Re: [PATCH 3/3] intel-iommu: PASID support

[Jason Wang]

On Fri, Jan 14, 2022 at 3:13 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Jan 14, 2022 at 01:58:07PM +0800, Jason Wang wrote:
> > > > Right, but I think you meant to do this only when scalable mode is 
> > > > disabled.
> > >
> > > Yes IMHO it will definitely suite for !scalable case since that's exactly 
> > > what
> > > we did before.  What I'm also wondering is even if scalable is enabled 
> > > but no
> > > "real" pasid is used, so if all the translations go through the default 
> > > pasid
> > > that stored in the device context entry, then maybe we can ignore 
> > > checking it.
> > > The latter is the "hacky" part mentioned above.
> >
> > The problem I see is that we can't know what PASID is used as default
> > without reading the context entry?
>
> Can the default NO_PASID being used in mixture of !NO_PASID use case on the
> same device?  If that's possible, then I agree..

My understanding is that it is possible.

>
> My previous idea should be based on the fact that if NO_PASID is used on one
> device, then all translations will be based on NO_PASID, but now I'm not sure
> of it.

Actually, what I meant is:

device 1 using transactions without PASID with RID2PASID 1
device 2 using transactions without PASID with RID2PASID 2

Then we can't assume a default pasid here.

>
> >
> > >
> > > The other thing to mention is, if we postpone the iotlb lookup to be after
> > > context entry, then logically we can have per-device iotlb, that means we 
> > > can
> > > replace IntelIOMMUState.iotlb with VTDAddressSpace.iotlb in the future, 
> > > too,
> > > which can also be more efficient.
> >
> > Right but we still need to limit the total slots and ATS is a better
> > way to deal with the IOTLB bottleneck actually.
>
> I think it depends on how the iotlb ghash is implemented.  Logically I think 
> if
> we can split the cache to per-device it'll be slightly better because we don't
> need to iterate over iotlbs of other devices when lookup anymore; meanwhile
> each iotlb takes less space too (no devfn needed anymore).

So we've already used sid in the IOTLB hash, I wonder how much we can
gain form this.

Thanks

>
> Thanks,
>
> --
> Peter Xu
>