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

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Date:   Thu, 2 Nov 2023 08:51:55 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Kai Huang <kai.huang@...el.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
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        Wei W Wang <wei.w.wang@...el.com>
Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report
 faults to userspace

On Thu, Nov 02, 2023, Xiaoyao Li wrote:
> 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@google.com
> 
> 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");