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

[Jason Gunthorpe]

On Thu, Oct 28, 2021 at 02:07:46AM +0000, Tian, Kevin wrote:
> > 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?

IMHO if the wbinvd is just a thought experiment then userspace should
directly control the wbinvd enable and present the iommufd as 'proof'
to enable it.

The thought experiment tells us that iommufd should have a wbinvd
ioctl, even if we don't implement it today. So access to iommufd is
also access to wbinvd.

iommufd should control/report via intel sepecific areas if the IOS is
no-snoop blocking or not so userspace can decide what it wants to do.

Jason