RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Tuesday, October 26, 2021 7:35 AM
> 
> On Fri, Oct 22, 2021 at 03:08:06AM +0000, Tian, Kevin wrote:
> 
> > > I have no idea what security model makes sense for wbinvd, that is the
> > > major question you have to answer.
> >
> > wbinvd flushes the entire cache in local cpu. It's more a performance
> > isolation problem but nothing can prevent it once the user is allowed
> > to call this ioctl. This is the main reason why wbinvd is a privileged
> > instruction and is emulated by kvm as a nop unless an assigned device
> > has no-snoop requirement. alternatively the user may call clflush
> > which is unprivileged and can invalidate a specific cache line, though
> > not efficient for flushing a big buffer.
> >
> > One tricky thing is that the process might be scheduled to different
> > cpus between writing buffers and calling wbinvd ioctl. Since wbvind
> > only has local behavior, it requires the ioctl to call wbinvd on all
> > cpus that this process has previously been scheduled on.
> 
> That is such a hassle, you may want to re-open this with the kvm
> people as it seems ARM also has different behavior between VM and
> process here.
> 
> The ideal is already not being met, so maybe we can keep special
> casing cache ops?
> 

Now Paolo confirmed wbinvd ioctl is just a thought experiment. 

Then Jason, want to have a clarification on 'keep special casing' here.

Did you mean adopting the vfio model which neither allows the user
to decide no-snoop format nor provides a wbinvd ioctl for the user
to manage buffers used for no-snoop traffic, or still wanting the user 
to decide no-snoop format but not implementing a wbinvd ioctl?

The latter option sounds a bit incomplete from uAPI p.o.v. but it
allows us to stay with one-format-one-ioas policy. And anyway the
userspace can still call clflush to do cacheline-based invalidation,
if necessary.

The former option would force us to support multi-formats-one-ioas.

either case it's iommufd which decides and tells kvm whether wbinvd 
is allowed for a process. 

Thanks
Kevin