From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
linux-kernel@vger.kernel.org,
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>,
Maxim Levitsky <mlevitsk@redhat.com>,
kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: X86: fix tlb_flush_guest()
Date: Thu, 27 May 2021 16:14:36 +0000 [thread overview]
Message-ID: <YK/FbFzKhZEmI40C@google.com> (raw)
In-Reply-To: <YK/FGYejaIu6EzSn@google.com>
+Maxim for real this time...
On Thu, May 27, 2021, Sean Christopherson wrote:
> +Maxim - A proper fix for this bug might fix your shadow paging + win10 boot
> issue, this also affects the KVM_REQ_HV_TLB_FLUSH used for HyperV PV
> flushing.
>
> On Thu, May 27, 2021, Paolo Bonzini wrote:
> > On 27/05/21 04:39, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@linux.alibaba.com>
> > >
> > > For KVM_VCPU_FLUSH_TLB used in kvm_flush_tlb_multi(), the guest expects
> > > the hypervisor do the operation that equals to native_flush_tlb_global()
> > > or invpcid_flush_all() in the specified guest CPU.
> > >
> > > When TDP is enabled, there is no problem to just flush the hardware
> > > TLB of the specified guest CPU.
> > >
> > > But when using shadowpaging, the hypervisor should have to sync the
> > > shadow pagetable at first before flushing the hardware TLB so that
> > > it can truely emulate the operation of invpcid_flush_all() in guest.
> >
> > Can you explain why?
>
> KVM's unsync logic hinges on guest TLB flushes. For page permission modifications
> that require a TLB flush to take effect, e.g. making a writable page read-only,
> KVM waits until the guest explicitly does said flush to propagate the changes to
> the shadow page tables. E.g. failure to sync PTEs could result in a read-only 4k
> page being writable when the guest expects it to be read-only.
>
> > Also it is simpler to handle this in kvm_vcpu_flush_tlb_guest, using "if
> > (tdp_enabled). This provides also a single, good place to add a comment
> > with the explanation of what invalid entries KVM_REQ_RELOAD is presenting.
>
> Ya.
>
> KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely
> offset the performance gains of the paravirtualized flush.
>
> And making a request won't work without revamping the order of request handling
> in vcpu_enter_guest(), e.g. KVM_REQ_MMU_RELOAD and KVM_REQ_MMU_SYNC are both
> serviced before KVM_REQ_STEAL_UPDATE.
>
> Cleaning up and documenting the MMU related requests is on my todo list, but the
> immediate fix should be tiny and I can do my cleanups on top.
>
> I believe the minimal fix is:
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 81ab3b8f22e5..b0072063f9bf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3072,6 +3072,9 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
> static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
> {
> ++vcpu->stat.tlb_flush;
> +
> + if (!tdp_enabled)
> + kvm_mmu_sync_roots(vcpu);
> static_call(kvm_x86_tlb_flush_guest)(vcpu);
> }
>
>
next prev parent reply other threads:[~2021-05-27 16:15 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-27 2:39 [PATCH] KVM: X86: fix tlb_flush_guest() Lai Jiangshan
2021-05-27 12:55 ` Paolo Bonzini
2021-05-27 16:13 ` Sean Christopherson
2021-05-27 16:14 ` Sean Christopherson [this message]
2021-05-27 19:28 ` Sean Christopherson
2021-05-28 1:13 ` Lai Jiangshan
2021-06-02 15:09 ` Sean Christopherson
2021-06-02 22:07 ` Sean Christopherson
2021-05-28 0:18 ` Lai Jiangshan
2021-05-28 0:26 ` Sean Christopherson
2021-05-28 1:29 ` Lai Jiangshan
2021-06-02 15:01 ` Sean Christopherson
2021-06-02 8:13 ` Lai Jiangshan
2021-05-29 22:12 ` Maxim Levitsky
2021-05-31 17:22 ` [PATCH V2] " Lai Jiangshan
2021-06-02 15:39 ` Sean Christopherson
2021-06-07 22:38 ` Maxim Levitsky
2021-06-08 0:03 ` Sean Christopherson
2021-06-08 14:01 ` Lai Jiangshan
2021-06-08 17:36 ` Paolo Bonzini
2021-06-08 21:31 ` Maxim Levitsky
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=YK/FbFzKhZEmI40C@google.com \
--to=seanjc@google.com \
--cc=bp@alien8.de \
--cc=hpa@zytor.com \
--cc=jiangshanlai@gmail.com \
--cc=jmattson@google.com \
--cc=joro@8bytes.org \
--cc=kvm@vger.kernel.org \
--cc=laijs@linux.alibaba.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=mlevitsk@redhat.com \
--cc=pbonzini@redhat.com \
--cc=tglx@linutronix.de \
--cc=vkuznets@redhat.com \
--cc=wanpengli@tencent.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).