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

[Peter Xu]

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 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.

> 
> >
> > 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).

Thanks,

-- 
Peter Xu