On Thu, Nov 02, 2023, Xiaoyao Li wrote:

On 11/2/2023 1:36 AM, Sean Christopherson wrote:

quoted

quoted

KVM_CAP_MEMORY_FAULT_INFO is x86 only, is it better to put this function to
<asm/kvm_host.h>?

I'd prefer to keep it in generic code, as it's highly likely to end up there
sooner than later.  There's a known use case for ARM (exit to userspace on missing
userspace mapping[*]), and I'm guessing pKVM (also ARM) will also utilize this API.

[*]https://lore.kernel.org/all/20230908222905.1321305-8-amoorthy@google.com (local)

I wonder how this CAP is supposed to be checked in userspace, for guest
memfd case? 

It's basically useless for guest_memfd.

	if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO) &&
	    run->exit_reason == KVM_EXIT_MEMORY_FAULT)
		abort("unexpected KVM_EXIT_MEMORY_FAULT");

In my implementation of QEMU patches, I find it's unnecessary. When
userspace gets an exit with KVM_EXIT_MEMORY_FAULT, it implies
"KVM_CAP_MEMORY_FAULT_INFO".

So I don't see how it is necessary in this series. Whether it's necessary or
not for [*], I don't have the answer but we can leave the discussion to that
patch series.

It's not strictly necessary there either.

However, Oliver felt (and presumably still feels) quite strongly, and I agree,
that neither reporting extra information shouldn't be tightly coupled to
KVM_CAP_EXIT_ON_MISSING or KVM_CAP_GUEST_MEMFD.

E.g. if userspace develops a "standalone" use case for KVM_CAP_MEMORY_FAULT_INFO,
userspace should be able to check for support without having to take a dependency
on KVM_CAP_GUEST_MEMFD, especially since because KVM_CAP_GUEST_MEMFD may not be
supported, i.e. userspace should be able to do:

	if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO))
		abort("KVM_CAP_MEMORY_FAULT_INFO required for fancy feature XYZ");