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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Fri, 10 Nov 2017 14:07:26 +0800
Message-ID: <CANRm+CxpcH5e1pspXSA6TgU-XCHntKHbg3R8iAyn5PnqFQsiWg@mail.gmail.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set
To:     Radim Krcmar <rkrcmar@redhat.com>
Cc:     Eduardo Valentin <eduval@amazon.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Matt Wilson <msw@amazon.com>, Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Waiman Long <longman@redhat.com>, kvm <kvm@vger.kernel.org>,
        linux-doc@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Jan H . Schoenherr" <jschoenh@amazon.de>,
        Anthony Liguori <aliguori@amazon.com>
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2017-11-10 0:00 GMT+08:00 Radim Krcmar <rkrcmar@redhat.com>:
> 2017-11-09 20:43+0800, Wanpeng Li:
>> 2017-11-07 4:26 GMT+08:00 Eduardo Valentin <eduval@amazon.com>:
>> > Currently, the existing qspinlock implementation will fallback to
>> > test-and-set if the hypervisor has not set the PV_UNHALT flag.
>> >
>> > This patch gives the opportunity to guest kernels to select
>> > between test-and-set and the regular queueu fair lock implementation
>> > based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
>> > flag is not set, the code will still fall back to test-and-set,
>> > but when the PV_DEDICATED flag is set, the code will use
>> > the regular queue spinlock implementation.
>> >
>> > With this patch, when in autoselect mode, the guest will
>> > use the default spinlock implementation based on host feature
>> > flags as follows:
>> >
>> > PV_DEDICATED =3D 1, PV_UNHALT =3D anything: default is qspinlock
>> > PV_DEDICATED =3D 0, PV_UNHALT =3D 1: default is pvqspinlock
>> > PV_DEDICATED =3D 0, PV_UNHALT =3D 0: default is tas
>> >
>> > Cc: Paolo Bonzini <pbonzini@redhat.com>
>> > Cc: "Radim Kr=C4=8Dm=C3=A1=C5=99" <rkrcmar@redhat.com>
>> > Cc: Jonathan Corbet <corbet@lwn.net>
>> > Cc: Thomas Gleixner <tglx@linutronix.de>
>> > Cc: Ingo Molnar <mingo@redhat.com>
>> > Cc: "H. Peter Anvin" <hpa@zytor.com>
>> > Cc: x86@kernel.org
>> > Cc: Peter Zijlstra <peterz@infradead.org>
>> > Cc: Waiman Long <longman@redhat.com>
>> > Cc: kvm@vger.kernel.org
>> > Cc: linux-doc@vger.kernel.org
>> > Cc: linux-kernel@vger.kernel.org
>> > Cc: Jan H. Schoenherr <jschoenh@amazon.de>
>> > Cc: Anthony Liguori <aliguori@amazon.com>
>> > Suggested-by: Matt Wilson <msw@amazon.com>
>> > Signed-off-by: Eduardo Valentin <eduval@amazon.com>
>> > ---
>>
>> You should also add a cpuid flag in kvm part.
>
> It is better without that.  The flag has no dependency on KVM (kernel
> hypervisor) code.

Do you mean -cpu host, +xxxx=EF=BC=8CI think it will result in "warning: ho=
st
doesn't support requested feature: CPUID.40000001H:eax.XXXXXX"

Regards,
Wanpeng Li

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