Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

[Xiaoyao Li]

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

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

	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.