linux-kernel - Re: [PATCH] KVM: X86: reset and read st->preempted in atomic way

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Message-ID: <YL668lnelAVNlLWx@google.com>
Date:   Tue, 8 Jun 2021 00:33:54 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: X86: reset and read st->preempted in atomic way

Nit: the shortlog is somewhat inaccurate now, maybe just:

  KVM: x86: Ensure PV TLB flush tracepoint reflects KVM behavior

or something along those lines.  Not sure what the best wording is :-/

On Tue, Jun 01, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> In record_steal_time(), st->preempted is read twice, and
> trace_kvm_pv_tlb_flush() might output result inconsistent if
> kvm_vcpu_flush_tlb_guest() see a different st->preempted later.
> 
> It is a very trivial problem and hardly has actual harm and can be
> avoided by reseting and reading st->preempted in atomic way via xchg().
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>

I saw this quirk too, but couldn't quite bring myself to care enought to test a
patch :-)

Reviewed-by: Sean Christopherson <seanjc@...gle.com>