linux-kernel - Re: [PATCH 3/4] KVM: X86: Use smp_rmb() to pair with smp_wmb() in mmu_try_to_unsync

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Message-ID: <4192556d-8890-d94b-d03d-23922ff1674c@linux.alibaba.com>
Date:   Thu, 21 Oct 2021 10:32:30 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Junaid Shahid <junaids@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.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>
Subject: Re: [PATCH 3/4] KVM: X86: Use smp_rmb() to pair with smp_wmb() in
 mmu_try_to_unsync_pages()

Hello Junaid Shahid

Any comments on the patch?  I may have been misunderstanding many things and I
have been often.

thanks
Lai

On 2021/10/19 19:01, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.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.
> 
> Cc: Junaid Shahid <junaids@...gle.com>
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>