linux-kernel - Re: [RFC PATCH v12 07/33] KVM: Add KVM_EXIT_MEMORY

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Date:   Tue, 3 Oct 2023 15:59:15 -0700
From:   Anish Moorthy <amoorthy@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>, kvm@...r.kernel.org,
        kvmarm@...ts.linux.dev, kvm-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>
Subject: Re: [RFC PATCH v12 07/33] KVM: Add KVM_EXIT_MEMORY_FAULT exit to
 report faults to userspace

On Mon, Oct 2, 2023 at 6:43 PM Sean Christopherson <seanjc@...gle.com> 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 giving userspace
> debug information.  All I'm proposing is that we would firmly state in the
> 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 (sadly).

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: