linux-kernel - Re: [PATCH V2] kvm x86/mmu: use KVM_REQ_MMU

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Message-ID: <871rjkp8rc.fsf@vitty.brq.redhat.com>
Date:   Wed, 02 Sep 2020 16:12:55 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...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 V2] kvm x86/mmu: use KVM_REQ_MMU_SYNC to sync when needed

Lai Jiangshan <jiangshanlai@...il.com> writes:

> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>
> When kvm_mmu_get_page() gets a page with unsynced children, the spt
> pagetable is unsynchronized with the guest pagetable. But the
> guest might not issue a "flush" operation on it when the pagetable
> entry is changed from zero or other cases. The hypervisor has the 
> responsibility to synchronize the pagetables.
>
> The linux kernel behaves as above for many years, But
> 8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)

Nit: checkpatch.pl complains here with 

ERROR: Please use git commit description style 'commit <12+ chars of
sha1> ("<title line>")' - ie: 'commit 8c8560b83390 ("KVM: x86/mmu: Use
KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes")'
#118: 
8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)

> inadvertently included a line of code to change it without giving any
> reason in the changelog. It is clear that the commit's intention was to
> change KVM_REQ_TLB_FLUSH -> KVM_REQ_TLB_FLUSH_CURRENT, so we don't
> unneedlesly flush other contexts but one of the hunks changed
> nearby KVM_REQ_MMU_SYNC instead.
>
> The this patch changes it back.
>
> Link: https://lore.kernel.org/lkml/20200320212833.3507-26-sean.j.christopherson@intel.com/
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
> Changed from v1:
> 	update patch description
>
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4e03841f053d..9a93de921f2b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2468,7 +2468,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  		}
>  
>  		if (sp->unsync_children)
> -			kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> +			kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
>  
>  		__clear_sp_write_flooding_count(sp);

FWIW, 

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

but it'd be great to hear from Sean).

-- 
Vitaly