From: Sean Christopherson <seanjc@google.com>
To: Lai Jiangshan <laijs@linux.alibaba.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Lai Jiangshan" <jiangshanlai@gmail.com>,
linux-kernel@vger.kernel.org,
"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>,
"Radim Krčmář" <rkrcmar@redhat.com>,
kvm@vger.kernel.org, "Maxim Levitsky" <mlevitsk@redhat.com>
Subject: Re: [PATCH] KVM: X86: fix tlb_flush_guest()
Date: Fri, 28 May 2021 00:26:13 +0000 [thread overview]
Message-ID: <YLA4peMjgeVvKlEn@google.com> (raw)
In-Reply-To: <d96f8c11-19e6-2c2d-91ff-6a7a51fa1b9c@linux.alibaba.com>
On Fri, May 28, 2021, Lai Jiangshan wrote:
>
> On 2021/5/28 00:13, Sean Christopherson wrote:
> > 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.
>
> Yes, it just fixes the said problem in the simplest way.
> I copied KVM_REQ_MMU_RELOAD from kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).
> (If the guest is not preempted, it will call invpcid_flush_all() and will be handled
> by this way)
The problem is that record_steal_time() is called after KVM_REQ_MMU_RELOAD
in vcpu_enter_guest() and so the reload request won't be recognized until the
next VM-Exit. It works for kvm_handle_invpcid() because vcpu_enter_guest() is
guaranteed to run between the invcpid code and VM-Enter.
> The improvement code will go later, and will not be backported.
I would argue that introducing a potential performance regression is in itself a
bug. IMO, going straight to kvm_mmu_sync_roots() is not high risk.
> The proper way to flush guest is to use code in
>
> https://lore.kernel.org/lkml/20210525213920.3340-1-jiangshanlai@gmail.com/
> as:
> + kvm_mmu_sync_roots(vcpu);
> + kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); //or just call flush_current directly
> + for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
> + vcpu->arch.mmu->prev_roots[i].need_sync = true;
>
> If need_sync patch is not accepted, we can just use kvm_mmu_sync_roots(vcpu)
> to keep the current pagetable and use kvm_mmu_free_roots() to free all the other
> roots in prev_roots.
I like the idea, I just haven't gotten around to reviewing that patch yet.
> > 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);
>
> it doesn't handle prev_roots which are also needed as
> shown in kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).
Ya, I belated realized this :-)
> > static_call(kvm_x86_tlb_flush_guest)(vcpu);
>
> For tdp_enabled, I think it is better to use kvm_x86_tlb_flush_current()
> to make it consistent with other shadowpage code.
>
> > }
> >
next prev parent reply other threads:[~2021-05-28 0:26 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
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 [this message]
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=YLA4peMjgeVvKlEn@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=rkrcmar@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).