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

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

On Sun, 11 Feb 2024 23:17:05 +0530
<ankita@nvidia.com> wrote:

> From: Ankit Agrawal <ankita@nvidia.com>
> 
> The code to map the MMIO in S2 as NormalNC is enabled when conveyed
> that the device is WC safe using a new flag VM_ALLOW_ANY_UNCACHED.
> 
> Make vfio-pci set the VM_ALLOW_ANY_UNCACHED flag.
> 
> This could be extended to other devices in the future once that
> is deemed safe.
> 
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Acked-by: Jason Gunthorpe <jgg@nvidia.com>
> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 1cbc990d42e0..eba2146202f9 100644
> --- 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.

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?

Thanks,
Alex

> -	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> +	vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP |
> +			VM_DONTEXPAND | VM_DONTDUMP);
>  	vma->vm_ops = &vfio_pci_mmap_ops;
>  
>  	return 0;