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From:   Anish Moorthy <amoorthy@google.com>
Date:   Tue, 3 Oct 2023 15:59:15 -0700
Message-ID: <CAF7b7mqyU059YpBBVYjTMNXf9VHSc6tbKrQ8avFXYtP6LWMh8Q@mail.gmail.com>
Subject: Re: [RFC PATCH v12 07/33] KVM: Add KVM_EXIT_MEMORY_FAULT exit to
 report faults to userspace
To:     Sean Christopherson <seanjc@google.com>
Cc:     Xiaoyao Li <xiaoyao.li@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
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        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
        Xu Yilun <yilun.xu@intel.com>,
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        Quentin Perret <qperret@google.com>,
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On Mon, Oct 2, 2023 at 6:43=E2=80=AFPM Sean Christopherson <seanjc@google.c=
om> wrote:
>
> > - I should go drop the patches annotating kvm_vcpu_read/write_page
> > from my series
>
> Hold up on that.  I'd prefer to keep them as there's still value in givin=
g userspace
> debug information.  All I'm proposing is that we would firmly state in th=
e
> documentation that those paths must be treated as informational-only.

Userspace would then need to know whether annotations were performed
from reliable/unreliable paths though, right? That'd imply another
flag bit beyond the current R/W/E bits.

> > - The helper function [a] for filling the memory_fault field
> > (downgraded back into the current union) can drop the "has the field
> > already been filled?" check/WARN.
>
> That would need to be dropped regardless because it's user-triggered (sad=
ly).

Well the current v5 of the series uses a non-userspace visible canary-
it seems like there'd still be value in that if we were to keep the
annotations in potentially unreliable spots. Although perhaps that
test failure you noticed [1] is a good counter-argument, since it
shows a known case where a current flow does multiple writes to the
memory_fault member.

[1] https://lore.kernel.org/all/202309141107.30863e9d-oliver.sang@intel.com

> Anyways, don't do anything just yet.

:salutes: