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From: Sean Christopherson <seanjc@google.com>
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Lai Jiangshan <laijs@linux.alibaba.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>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Avi Kivity <avi@redhat.com>,
	kvm@vger.kernel.org
Subject: Re: [PATCH 1/7] KVM: X86: Fix missed remote tlb flush in rmap_write_protect()
Date: Thu, 2 Sep 2021 21:38:26 +0000	[thread overview]
Message-ID: <YTFEUmrjcyI9V1z9@google.com> (raw)
In-Reply-To: <20210824075524.3354-2-jiangshanlai@gmail.com>

On Tue, Aug 24, 2021, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.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")

Should be

Fixes: a4ee1ca4a36e ("KVM: MMU: delay flush all tlbs on sync_page path")

> Signed-off-by: Lai Jiangshan <laijs@linux.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;
>  	}
>  
>  	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);
>  	}

My vote is to do a revert of a4ee1ca4a36e with slightly less awful batching, and
then improve the batching even further if there's a noticeable loss of performance
(or just tell people to stop using shadow paging :-D).  Zapping SPTEs but not
flushing is just asking for these types of whack-a-mole bugs.

E.g. instead of a straight revert, do this for sync_page():

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 50ade6450ace..1fca27a08c00 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -1095,13 +1095,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
                        return 0;

                if (FNAME(prefetch_invalid_gpte)(vcpu, sp, &sp->spt[i], gpte)) {
-                       /*
-                        * Update spte before increasing tlbs_dirty to make
-                        * sure no tlb flush is lost after spte is zapped; see
-                        * the comments in kvm_flush_remote_tlbs().
-                        */
-                       smp_wmb();
-                       vcpu->kvm->tlbs_dirty++;
+                       set_spte_ret |= SET_SPTE_NEED_REMOTE_TLB_FLUSH;
                        continue;
                }

@@ -1116,12 +1110,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)

                if (gfn != sp->gfns[i]) {
                        drop_spte(vcpu->kvm, &sp->spt[i]);
-                       /*
-                        * The same as above where we are doing
-                        * prefetch_invalid_gpte().
-                        */
-                       smp_wmb();
-                       vcpu->kvm->tlbs_dirty++;
+                       set_spte_ret |= SET_SPTE_NEED_REMOTE_TLB_FLUSH;
                        continue;
                }



  reply	other threads:[~2021-09-02 21:38 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24  7:55 [PATCH 0/7] KVM: X86: MMU: misc fixes and cleanups Lai Jiangshan
2021-08-24  7:55 ` [PATCH 1/7] KVM: X86: Fix missed remote tlb flush in rmap_write_protect() Lai Jiangshan
2021-09-02 21:38   ` Sean Christopherson [this message]
2021-09-13  9:57   ` Maxim Levitsky
2021-08-24  7:55 ` [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it Lai Jiangshan
2021-09-02 23:40   ` Sean Christopherson
2021-09-02 23:54     ` Sean Christopherson
2021-09-03  0:44       ` Lai Jiangshan
2021-09-03 16:06         ` Sean Christopherson
2021-09-03 16:25           ` Lai Jiangshan
2021-09-03 16:40             ` Sean Christopherson
2021-09-03 17:00               ` Lai Jiangshan
2021-09-03 16:33           ` Lai Jiangshan
2021-09-03  0:51     ` Lai Jiangshan
2021-09-13 11:30     ` Maxim Levitsky
2021-09-13 20:49       ` Sean Christopherson
2021-09-13 22:31         ` Maxim Levitsky
2021-08-24  7:55 ` [PATCH 3/7] KVM: X86: Zap the invalid list after remote tlb flushing Lai Jiangshan
2021-09-02 21:54   ` Sean Christopherson
2021-08-24  7:55 ` [PATCH 4/7] KVM: X86: Remove FNAME(update_pte) Lai Jiangshan
2021-09-13  9:49   ` Maxim Levitsky
2021-08-24  7:55 ` [PATCH 5/7] KVM: X86: Don't unsync pagetables when speculative Lai Jiangshan
2021-09-13 11:02   ` Maxim Levitsky
2021-09-18  3:06     ` Lai Jiangshan
2021-08-24  7:55 ` [PATCH 6/7] KVM: X86: Don't check unsync if the original spte is writible Lai Jiangshan
2021-08-24  7:55 ` [PATCH 7/7] KVM: X86: Also prefetch the last range in __direct_pte_prefetch() Lai Jiangshan
2021-08-25 15:18   ` Sean Christopherson
2021-08-25 22:58     ` Lai Jiangshan
2021-08-31 18:02 ` [PATCH 0/7] KVM: X86: MMU: misc fixes and cleanups Lai Jiangshan
2021-08-31 21:57   ` Sean Christopherson

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