linux-kernel - Re: [PATCH] KVM: x86/mmu: Don't rebuild page when the page is synced and no tlb flushing is required

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Date:   Wed, 16 Mar 2022 10:19:33 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Hou Wenlong <houwenlong.hwl@...group.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Don't rebuild page when the page is synced
 and no tlb flushing is required

On Tue, Mar 15, 2022 at 6:45 PM Hou Wenlong <houwenlong.hwl@...group.com> wrote:
>
> Before Commit c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page()
> to return true when remote flush is needed"), the return value
> of kvm_sync_page() indicates whether the page is synced, and
> kvm_mmu_get_page() would rebuild page when the sync fails.
> But now, kvm_sync_page() returns false when the page is
> synced and no tlb flushing is required, which leads to
> rebuild page in kvm_mmu_get_page(). So return the return
> value of mmu->sync_page() directly and check it in
> kvm_mmu_get_page(). If the sync fails, the page will be
> zapped and the invalid_list is not empty, so set flush as
> true is accepted in mmu_sync_children().
>

Good catch.

Acked-by: Lai Jiangshan <jiangshanlai@...il.com>

> Fixes: c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed")
> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3b8da8b0745e..8efd165ee27c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1866,17 +1866,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>           &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)])     \
>                 if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
>
> -static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> +static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>                          struct list_head *invalid_list)

The comments for FNAME(sync_page) can be copied here.