Re: [PATCH v7 2/4] mm: introduce new flag to indicate wc safe

[David Hildenbrand <david@redhat.com>]

On 11.02.24 18:47, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> Generalizing S2 setting from DEVICE_nGnRE to NormalNc for non PCI
> devices may be problematic. E.g. GICv2 vCPU interface, which is
> effectively a shared peripheral, can allow a guest to affect another
> guest's interrupt distribution. The issue may be solved by limiting
> the relaxation to mappings that have a user VMA. Still there is
> insufficient information and uncertainity in the behavior of

s/uncertainity/uncertainty/

> non PCI drivers.
> 
> Add a new flag VM_ALLOW_ANY_UNCACHED to indicate KVM that the device
> is WC capable and these S2 changes can be extended to it. KVM can use
> this flag to activate the code.
> 

MM people will stumble only over this commit at some point, looking for 
details. It might make sense to add a bit more details on the underlying 
problem (user space tables vs. stage-1 vs. stage-2) and why we want to 
have a different mapping in user space compared to stage-1.

Then, describe that the VMA flag was found to be the simplest and 
cleanest way to communicate this information from VFIO to KVM.

> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
>   include/linux/mm.h | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f5a97dec5169..59576e56c58b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -391,6 +391,20 @@ extern unsigned int kobjsize(const void *objp);
>   # define VM_UFFD_MINOR		VM_NONE
>   #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
>   
> +/*
> + * This flag is used to connect VFIO to arch specific KVM code. It
> + * indicates that the memory under this VMA is safe for use with any
> + * non-cachable memory type inside KVM. Some VFIO devices, on some
> + * platforms, are thought to be unsafe and can cause machine crashes
> + * if KVM does not lock down the memory type.
> + */
> +#ifdef CONFIG_64BIT
> +#define VM_ALLOW_ANY_UNCACHED_BIT	39
> +#define VM_ALLOW_ANY_UNCACHED		BIT(VM_ALLOW_ANY_UNCACHED_BIT)
> +#else
> +#define VM_ALLOW_ANY_UNCACHED		VM_NONE
> +#endif
> +
>   /* Bits set in the VMA until the stack is in its final location */
>   #define VM_STACK_INCOMPLETE_SETUP (VM_RAND_READ | VM_SEQ_READ | VM_STACK_EARLY)
>   

It's not perfect (very VFIO <-> KVM specific right now, VMA flags feel a 
bit wrong), but it certainly easier and cleaner than any alternatives I 
could think of.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb