linux-kernel - Re: [PATCH v2 21/43] KVM: VMX: Clean up PI pre/post-block WARNs

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Message-ID: <YXrC/X6b+tgn6cJ9@google.com>
Date:   Thu, 28 Oct 2021 15:34:21 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 21/43] KVM: VMX: Clean up PI pre/post-block WARNs

On Thu, Oct 28, 2021, Maxim Levitsky wrote:
> On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> > Move the WARN sanity checks out of the PI descriptor update loop so as
> > not to spam the kernel log if the condition is violated and the update
> > takes multiple attempts due to another writer.  This also eliminates a
> > few extra uops from the retry path.
> > 
> > Technically not checking every attempt could mean KVM will now fail to
> > WARN in a scenario that would have failed before, but any such failure
> > would be inherently racy as some other agent (CPU or device) would have
> > to concurrent modify the PI descriptor.

...

> Don't know for sure if this is desired. I'll would just use WARN_ON_ONCE instead
> if the warning spams the log.
> 
> If there is a race I would rather want to catch it even if rare.

Paolo had similar concerns[*].  I copied the most relevant part of the discussion
below, let me know if you object to the outcome.

Thanks for the reviews!

[*] https://lore.kernel.org/all/YXllGfrjPX1pVUx6@google.com/T/#u

On Wed, Oct 27, 2021 at 8:38 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> On 27/10/21 17:28, Sean Christopherson wrote:
> > On Wed, Oct 27, 2021, Paolo Bonzini wrote:
> > > On 27/10/21 16:41, Sean Christopherson wrote:
> > > > The other thing I don't like about having the WARN in the loop is that it suggests
> > > > that something other than the vCPU can modify the NDST and SN fields, which is
> > > > wrong and confusing (for me).
> > >
> > > Yeah, I can agree with that.  Can you add it in a comment above the cmpxchg
> > > loop, it can be as simple as
> > >
> > > 	/* The processor can set ON concurrently.  */
> > >
> > > when you respin patch 21 and the rest of the series?
> >
> > I can definitely add a comment, but I think that comment is incorrect.
>
> It's completely backwards indeed.  I first had "the hardware" and then
> shut down my brain for a second to replace it.
>
> > So something like this?
> >
> > 	/* ON can be set concurrently by a different vCPU or by hardware. */
>
> Yes, of course.