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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Tue, 28 May 2019 14:11:04 +0800
Message-ID: <CANRm+CwnJoj0EwWoFC44SXVUTLdE+iFGovaMr4Yf=OzbaW36sA@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: add support for user wait instructions
To:     Tao Xu <tao3.xu@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        kvm <kvm@vger.kernel.org>, linux-doc@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, jingqi.liu@intel.com
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On Tue, 28 May 2019 at 13:16, Tao Xu <tao3.xu@intel.com> wrote:
>
>
> On 27/05/2019 18:30, Peter Zijlstra wrote:
> > On Fri, May 24, 2019 at 03:56:35PM +0800, Tao Xu wrote:
> >> This patch adds support for UMONITOR, UMWAIT and TPAUSE instructions
> >> in kvm, and by default dont't expose it to kvm and provide a capability
> >> to enable it.
> >
> > I'm thinking this should be conditional on the guest being a 1:1 guest,
> > and I also seem to remember we have bits for that already -- they were
> > used to disable paravirt spinlocks for example.
> >
>
> Hi Peter,
>
> I am wondering if "1:1 guest" means different guests in the same host
> should have different settings on user wait instructions?
>
> User wait instructions(UMONITOR, UMWAIT and TPAUSE) can use in guest
> only when the VMCS Secondary Processor-Based VM-Execution Control bit 26
> is 1, otherwise any execution of TPAUSE, UMONITOR, or UMWAIT causes a #UD.
>
> So with a capability to enable it, we use qemu kvm_vm_ioctl_enable_cap()
> to enable it. The qemu link is blew:
> https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg05810.html
>
> By using different QEMU parameters, different guests in the same host
> would have different features with or without user wait instructions.
>
> About "disable paravirt spinlocks" case, I am wondering if it uses

Please refer to a4429e53c9 (KVM: Introduce paravirtualization hints
and KVM_HINTS_DEDICATED) and b2798ba0b87 (KVM: X86: Choose qspinlock
when dedicated physical CPUs are available).

> kernel parameters? If it uses kernel parameters, different guests in the
> same host may have same settings on user wait instructions.
>
> Or when we uses kernel parameters to disable user wait instructions, for
> a host chooses to enable user wait instructions, we should do some work
> on QEMU to choose disable or enable user wait instructions?
>
> Thanks
>
> Tao