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Date:   Thu, 21 Oct 2021 19:44:40 +0200
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Subject: Re: [PATCH 3/4] KVM: X86: Use smp_rmb() to pair with smp_wmb() in
 mmu_try_to_unsync_pages()
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To:     Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        Junaid Shahid <junaids@google.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
References: <20211019110154.4091-1-jiangshanlai@gmail.com>
 <20211019110154.4091-4-jiangshanlai@gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211019110154.4091-4-jiangshanlai@gmail.com>
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On 19/10/21 13:01, Lai Jiangshan wrote:
> From: Lai Jiangshan<laijs@linux.alibaba.com>
> 
> The commit 578e1c4db2213 ("kvm: x86: Avoid taking MMU lock in
> kvm_mmu_sync_roots if no sync is needed") added smp_wmb() in
> mmu_try_to_unsync_pages(), but the corresponding smp_load_acquire()
> isn't used on the load of SPTE.W which is impossible since the load of
> SPTE.W is performed in the CPU's pagetable walking.
> 
> This patch changes to use smp_rmb() instead.  This patch fixes nothing
> but just comments since smp_rmb() is NOP and compiler barrier() is not
> required since the load of SPTE.W is before VMEXIT.

I think that even implicit loads during pagetable walking obey read-read 
ordering on x86, but this is clearer and it is necessary for patch 4.

Paolo