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

[Alex Williamson <alex.williamson@redhat.com>]

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