Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate_user

Nicolin Chen
2023-03-17T14:24:23Z

On Fri, Mar 17, 2023 at 09:41:34AM +0000, Tian, Kevin wrote:
> External email: Use caution opening links or attachments
>
>
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Saturday, March 11, 2023 12:20 AM
> >
> > What I'm broadly thinking is if we have to make the infrastructure for
> > VCMDQ HW accelerated invalidation then it is not a big step to also
> > have the kernel SW path use the same infrastructure just with a CPU
> > wake up instead of a MMIO poke.
> >
> > Ie we have a SW version of VCMDQ to speed up SMMUv3 cases without HW
> > support.
> >
>
> I thought about this in VT-d context. Looks there are some difficulties.
>
> The most prominent one is that head/tail of the VT-d invalidation queue
> are in MMIO registers. Handling it in kernel iommu driver suggests
> reading virtual tail register and updating virtual head register. Kind of
> moving some vIOMMU awareness into the kernel which, iirc, is not
> a welcomed model.

I had a similar question in another email:
"Firstly, the consumer and producer indexes might need
to be synced between the host and kernel?"
And Jason replied me with this:
"No, qemu would handles this. The kernel would just read the command
entries it is told by qemu to read which qemu has already sorted out."

Maybe there is no need of a concern for the head/tail readings?

> vhost doesn't have this problem as its vring structure fully resides in
> memory including ring tail/head. As long as kernel vhost driver understands
> the structure and can send/receive notification to/from kvm then the
> in-kernel acceleration works seamlessly.
>
> Not sure whether SMMU has similar obstacle as VT-d. But this is my
> impression why vhost-iommu is preferred when talking about such
> optimization before.

SMMU has a similar pair of head/tail pointers to the invalidation
queue (consumer/producer indexes and command queue in SMMU term).

Thanks
Nic