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From: Sean Christopherson <seanjc@google.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
	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 2/7] KVM: X86: Synchronize the shadow pagetable before link it
Date: Mon, 13 Sep 2021 20:49:29 +0000	[thread overview]
Message-ID: <YT+5WZsT4bfFSezR@google.com> (raw)
In-Reply-To: <0103c8b2cccea601bd3474f47d982b37e9536921.camel@redhat.com>

On Mon, Sep 13, 2021, Maxim Levitsky wrote:
> On Thu, 2021-09-02 at 23:40 +0000, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 4853c033e6ce..03293cd3c7ae 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -2143,8 +2143,10 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> >  			kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> >  		}
> > 
> > -		if (sp->unsync_children)
> > -			kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> > +		if (sp->unsync_children) {
> > +			kvm_make_all_cpus_request(KVM_REQ_MMU_SYNC, vcpu);
> 
> I don't know the KVM mmu well so I miss something here most likely,
> but why to switch to kvm_make_all_cpus_request?
> 
> MMU is shared by all VCPUs, and the process of its syncing should also do
> remote TLB flushes when needed?
> 
> Another thing I don't fully understand is why this patch is needed. If we
> link an SP which has unsync children, we raise KVM_REQ_MMU_SYNC, which I
> think means that *this* vCPU will sync the whole MMU on next guest entry,
> including these unsync child SPs. Could you explain this?

Answering all three questions at once, the problem is that KVM links in a new SP
that points at unsync'd SPs _before_ servicing KVM_REQ_MMU_SYNC.  While the vCPU
is guaranteed to service KVM_REQ_MMU_SYNC before entering the guest, that doesn't
hold true for other vCPUs.  As a result, there's a window where a different vCPU
can consume the stale, unsync SP via the new SP.

  reply	other threads:[~2021-09-13 20:49 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
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 [this message]
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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