Re: [PATCH v7 4/4] vfio: convey kvm that the vfio-pci device is wc safe

Alex Williamson
2024-02-12T17:27:24Z

On Mon, 12 Feb 2024 13:20:01 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Feb 12, 2024 at 10:05:02AM -0700, Alex Williamson wrote:
>
> > > --- a/drivers/vfio/pci/vfio_pci_core.c
> > > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > > @@ -1862,8 +1862,12 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma
> > > /*
> > > * See remap_pfn_range(), called from vfio_pci_fault() but we can't
> > > * change vm_flags within the fault handler. Set them now.
> > > + *
> > > + * Set an additional flag VM_ALLOW_ANY_UNCACHED to convey kvm that
> > > + * the device is wc safe.
> > > */
> >
> > That's a pretty superficial comment. Check that this is accurate, but
> > maybe something like:
> >
> > The VM_ALLOW_ANY_UNCACHED flag is implemented for ARM64,
> > allowing stage 2 device mapping attributes to use Normal-NC
> ^^^^
>
> > rather than DEVICE_nGnRE, which allows guest mappings
> > supporting combining attributes (WC). This attribute has
> > potential risks with the GICv2 VCPU interface, but is expected
> > to be safe for vfio-pci use cases.
>
> Sure, if you want to elaborate more
>
> The VM_ALLOW_ANY_UNCACHED flag is implemented for ARM64,
> allowing KVM stage 2 device mapping attributes to use Normal-NC
> rather than DEVICE_nGnRE, which allows guest mappings
> supporting combining attributes (WC). ARM does not architecturally
> guarentee this is safe, and indeed some MMIO regions like the GICv2
> VCPU interface can trigger uncontained faults if Normal-NC is used.
>
> Even worse we expect there are platforms where even DEVICE_nGnRE can
> allow uncontained faults in conercases. Unfortunately existing ARM
^^^^^^^^^^

*corner cases

> IP requires platform integration to take responsibility to prevent
> this.
>
> To safely use VFIO in KVM the platform must guarantee full safety
> in the guest where no action taken against a MMIO mapping can
> trigger an uncontainer failure. We belive that most VFIO PCI
> platforms support this for both mapping types, at least in common
> flows, based on some expectations of how PCI IP is integrated. This
> can be enabled more broadly, for instance into vfio-platform
> drivers, but only after the platform vendor completes auditing for
> safety.

I like it, please incorporate into the next version.

> > And specifically, I think these other devices that may be problematic
> > as described in the cover letter is a warning against use for
> > vfio-platform, is that correct?
>
> Maybe more like "we have a general consensus that vfio-pci is likely
> safe due to how PCI IP is typically integrated, but it is much less
> obvious for other VFIO bus types. As there is no known WC user for
> vfio-platform drivers be conservative and do not enable it."

Ok. Thanks for the clarification.

Alex