linux-kernel - Re: [PATCH v8 4/4] vfio: convey kvm that the vfio-pci device is wc safe

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Date: Thu, 22 Feb 2024 13:52:40 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: <ankita@...dia.com>
Cc: <jgg@...dia.com>, <maz@...nel.org>, <oliver.upton@...ux.dev>,
 <james.morse@....com>, <suzuki.poulose@....com>, <yuzenghui@...wei.com>,
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 <jhubbard@...dia.com>, <danw@...dia.com>, <kvmarm@...ts.linux.dev>,
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Subject: Re: [PATCH v8 4/4] vfio: convey kvm that the vfio-pci device is wc
 safe

On Tue, 20 Feb 2024 12:59:26 +0530
<ankita@...dia.com> wrote:

> From: Ankit Agrawal <ankita@...dia.com>
> 
> 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
> guarantee this is safe, and indeed some MMIO regions like the GICv2
> VCPU interface can trigger uncontained faults if Normal-NC is used.
> 
> 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 uncontained 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. So
> make vfio-pci set the VM_ALLOW_ANY_UNCACHED flag.
> 
> Suggested-by: Catalin Marinas <catalin.marinas@....com>
> Acked-by: Jason Gunthorpe <jgg@...dia.com>
> Acked-by: Catalin Marinas <catalin.marinas@....com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Ankit Agrawal <ankita@...dia.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 1cbc990d42e0..c93bea18fc4b 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1862,8 +1862,24 @@ 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_ALLOW_ANY_UNCACHED: The VMA 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 guarantee this is safe, and indeed some MMIO
> +	 * regions like the GICv2 VCPU interface can trigger uncontained
> +	 * faults if Normal-NC is used.
> +	 *
> +	 * 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 uncontained 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. So set VM_ALLOW_ANY_UNCACHED in VMA flags.
>  	 */
> -	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;

Acked-by: Alex Williamson <alex.williamson@...hat.com>