Re: [PATCH v3 0/6] Expose GPU memory as coherently CPU accessible

[Marc Zyngier]

On Wed, 12 Apr 2023 13:53:07 +0100,
Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> 
> On Wed, Apr 12, 2023 at 01:28:08PM +0100, Marc Zyngier wrote:
> > On Wed, 05 Apr 2023 19:01:28 +0100,
> > <ankita@xxxxxxxxxx> wrote:
> > > 
> > > From: Ankit Agrawal <ankita@xxxxxxxxxx>
> > > 
> > > NVIDIA's upcoming Grace Hopper Superchip provides a PCI-like device
> > > for the on-chip GPU that is the logical OS representation of the
> > > internal propritary cache coherent interconnect.
> > > 
> > > This representation has a number of limitations compared to a real PCI
> > > device, in particular, it does not model the coherent GPU memory
> > > aperture as a PCI config space BAR, and PCI doesn't know anything
> > > about cacheable memory types.
> > > 
> > > Provide a VFIO PCI variant driver that adapts the unique PCI
> > > representation into a more standard PCI representation facing
> > > userspace. The GPU memory aperture is obtained from ACPI, according to
> > > the FW specification, and exported to userspace as the VFIO_REGION
> > > that covers the first PCI BAR. qemu will naturally generate a PCI
> > > device in the VM where the cacheable aperture is reported in BAR1.
> > > 
> > > Since this memory region is actually cache coherent with the CPU, the
> > > VFIO variant driver will mmap it into VMA using a cacheable mapping.
> > > 
> > > As this is the first time an ARM environment has placed cacheable
> > > non-struct page backed memory (eg from remap_pfn_range) into a KVM
> > > page table, fix a bug in ARM KVM where it does not copy the cacheable
> > > memory attributes from non-struct page backed PTEs to ensure the guest
> > > also gets a cacheable mapping.
> > 
> > This is not a bug, but a conscious design decision. As you pointed out
> > above, nothing needed this until now, and a device mapping is the only
> > safe thing to do as we know exactly *nothing* about the memory that
> > gets mapped.
> 
> IMHO, from the mm perspective, the bug is using pfn_is_map_memory() to
> determine the cachability or device memory status of a PFN in a
> VMA. That is not what that API is for.

It is the right API for what KVM/arm64 has been designed for. RAM gets
a normal memory mapping, and everything else gets device. That may not
suit your *new* use case, but that doesn't make it broken.

> 
> The cachability should be determined by the pgprot bits in the VMA.
> 
> VM_IO is the flag that says the VMA maps memory with side-effects.
> 
> I understand in ARM KVM it is not allowed for the VM and host to have
> different cachability, so mis-detecting host cachable memory and
> making it forced non-cachable in the VM is not a safe thing to do?

Only if you insist on not losing coherency between the two aliases
used at the same time (something that would seem pretty improbable).
And said coherency can be restored by using CMOs, as documented in
B2.8.

	M.

-- 
Without deviation from the norm, progress is not possible.