linux-kernel - Re: [PATCH 1/7] KVM: X86: Fix missed remote tlb flush in rmap_write

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Date:   Mon, 13 Sep 2021 12:57:03 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <laijs@...ux.alibaba.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>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 1/7] KVM: X86: Fix missed remote tlb flush in
 rmap_write_protect()

On Tue, 2021-08-24 at 15:55 +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> When kvm->tlbs_dirty > 0, some rmaps might have been deleted
> without flushing tlb remotely after kvm_sync_page().  If @gfn
> was writable before and it's rmaps was deleted in kvm_sync_page(),
> we need to flush tlb too even if __rmap_write_protect() doesn't
> request it.
> 
> Fixes: 4731d4c7a077 ("KVM: MMU: out of sync shadow core")
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4853c033e6ce..313918df1a10 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1420,6 +1420,14 @@ bool kvm_mmu_slot_gfn_write_protect(struct kvm *kvm,
>  			rmap_head = gfn_to_rmap(gfn, i, slot);
>  			write_protected |= __rmap_write_protect(kvm, rmap_head, true);
>  		}
> +		/*
> +		 * When kvm->tlbs_dirty > 0, some rmaps might have been deleted
> +		 * without flushing tlb remotely after kvm_sync_page().  If @gfn
> +		 * was writable before and it's rmaps was deleted in kvm_sync_page(),
> +		 * we need to flush tlb too.
> +		 */
> +		if (min_level == PG_LEVEL_4K && kvm->tlbs_dirty)
> +			write_protected = true;

This is a bit misleading, as the gfn is not write protected in this case, but we
just want tlb flush. Maybe add a new output paramter to the function, telling
if we want a tlb flush?

>  	}
>  
>  	if (is_tdp_mmu_enabled(kvm))
> @@ -5733,6 +5741,14 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
>  		flush = slot_handle_level(kvm, memslot, slot_rmap_write_protect,
>  					  start_level, KVM_MAX_HUGEPAGE_LEVEL,
>  					  false);
> +		/*
> +		 * When kvm->tlbs_dirty > 0, some rmaps might have been deleted
> +		 * without flushing tlb remotely after kvm_sync_page().  If @gfn
> +		 * was writable before and it's rmaps was deleted in kvm_sync_page(),
> +		 * we need to flush tlb too.
> +		 */
> +		if (start_level == PG_LEVEL_4K && kvm->tlbs_dirty)
> +			flush = true;
>  		write_unlock(&kvm->mmu_lock);
>  	}
>  


Best regards,
	Maxim Levitsky