Re: [PATCH v5 04/17] KVM: Add KVM_CAP_MEMORY_FAULT_INFO

[David Matlack <dmatlack@google.com>]

On Fri, Sep 8, 2023 at 3:30 PM Anish Moorthy <amoorthy@google.com> wrote:
>
> KVM_CAP_MEMORY_FAULT_INFO allows kvm_run to return useful information
> besides a return value of -1 and errno of EFAULT when a vCPU fails an
> access to guest memory which may be resolvable by userspace.
>
> Add documentation, updates to the KVM headers, and a helper function
> (kvm_handle_guest_uaccess_fault()) for implementing the capability.
>
> Mark KVM_CAP_MEMORY_FAULT_INFO as available on arm64 and x86, even
> though EFAULT annotation are currently totally absent. Picking a point
> to declare the implementation "done" is difficult because
>
>   1. Annotations will be performed incrementally in subsequent commits
>      across both core and arch-specific KVM.
>   2. The initial series will very likely miss some cases which need
>      annotation. Although these omissions are to be fixed in the future,
>      userspace thus still needs to expect and be able to handle
>      unannotated EFAULTs.
>
> Given these qualifications, just marking it available here seems the
> least arbitrary thing to do.
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Anish Moorthy <amoorthy@google.com>
> ---
[...]
> +::
> +       union {
> +               /* KVM_SPEC_EXIT_MEMORY_FAULT */
> +               struct {
> +                       __u64 flags;
> +                       __u64 gpa;
> +                       __u64 len; /* in bytes */

I wonder if `gpa` and `len` should just be replaced with `gfn`.

- We don't seem to care about returning an exact `gpa` out to
userspace since this series just returns gpa = gfn * PAGE_SIZE out to
userspace.
- The len we return seems kind of arbitrary. PAGE_SIZE on x86 and
vma_pagesize on ARM64. But at the end of the day we're not asking the
kernel to fault in any specific length of mapping. We're just asking
for gfn-to-pfn for a specific gfn.
- I'm not sure userspace will want to do anything with this information.