Re: [PATCH v6 4/4] vfio: convey kvm that the vfio-pci device is wc safe

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

On Thu, 8 Feb 2024 02:16:52 +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_VFIO_ALLOW_WC.
> 
> Make vfio-pci set the VM_VFIO_ALLOW_WC flag.
> 
> This could be extended to other devices in the future once that
> is deemed safe.
> 
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Acked-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 1cbc990d42e0..c3f95ec7fc3a 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1863,7 +1863,8 @@ 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.
>  	 */
> -	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> +	vm_flags_set(vma, VM_VFIO_ALLOW_WC | VM_IO | VM_PFNMAP |
> +			VM_DONTEXPAND | VM_DONTDUMP);
>  	vma->vm_ops = &vfio_pci_mmap_ops;
>  
>  	return 0;

The comment above this is justifying the flags as equivalent to those
set by the remap_pfn_range() path.  That's no longer the case and the
additional flag needs to be described there.

I'm honestly surprised that a vm_flags bit named so specifically for a
single driver has gotten this far.  It seems like the vfio use case for
this and associated FUD for other use cases could all be encompassed in
the comment where the bit is defined and we could use a name like
VM_ALLOW_ANY_UNCACHED or VM_IO_ANY.  Thanks,

Alex