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

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Mar 17, 2023 at 09:47:47AM +0000, Tian, Kevin wrote:
> External email: Use caution opening links or attachments
> 
> 
> > From: Robin Murphy <robin.murphy@arm.com>
> > Sent: Thursday, March 9, 2023 10:49 PM
> > > +   case CMDQ_OP_ATC_INV:
> > > +           ssid = inv_info->ssid;
> > > +           iova = inv_info->range.start;
> > > +           size = inv_info->range.last - inv_info->range.start + 1;
> > > +           break;
> >
> > Can we do any better than multiplying every single ATC_INV command, even
> > for random bogus StreamIDs, into multiple commands across every physical
> > device? In fact, I'm not entirely confident this isn't problematic, if
> > the guest wishes to send invalidations for one device specifically while
> > it's put some other device into a state where sending it a command would
> > do something bad. At the very least, it's liable to be confusing if the
> > guest sends a command for one StreamID but gets an error back for a
> > different one.
> >
> 
> Or do we need support this cmd at all?
> 
> For vt-d we always implicitly invalidate ATC following a iotlb invalidation
> request from userspace. Then vIOMMU just treats it as a nop in the
> virtual queue.
> 
> IMHO a sane iommu driver should always invalidate both iotlb and atc
> together. I'm not sure a valid usage where iotlb is invalidated while
> atc is left with some stale mappings.

vSMMU code in QEMU actually doesn't forward this command. So,
I guess that you are right about this support here and we may
just drop it.

Thanks!
Nic