linux-kernel - Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Date:   Fri, 10 Nov 2017 14:07:26 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krcmar <rkrcmar@...hat.com>
Cc:     Eduardo Valentin <eduval@...zon.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Matt Wilson <msw@...zon.com>, Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>, kvm <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Jan H . Schoenherr" <jschoenh@...zon.de>,
        Anthony Liguori <aliguori@...zon.com>
Subject: Re: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when
 PV_DEDICATED is set

2017-11-10 0:00 GMT+08:00 Radim Krcmar <rkrcmar@...hat.com>:
> 2017-11-09 20:43+0800, Wanpeng Li:
>> 2017-11-07 4:26 GMT+08:00 Eduardo Valentin <eduval@...zon.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 = 1, PV_UNHALT = anything: default is qspinlock
>> > PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock
>> > PV_DEDICATED = 0, PV_UNHALT = 0: default is tas
>> >
>> > Cc: Paolo Bonzini <pbonzini@...hat.com>
>> > Cc: "Radim Krčmář" <rkrcmar@...hat.com>
>> > Cc: Jonathan Corbet <corbet@....net>
>> > Cc: Thomas Gleixner <tglx@...utronix.de>
>> > Cc: Ingo Molnar <mingo@...hat.com>
>> > Cc: "H. Peter Anvin" <hpa@...or.com>
>> > Cc: x86@...nel.org
>> > Cc: Peter Zijlstra <peterz@...radead.org>
>> > Cc: Waiman Long <longman@...hat.com>
>> > Cc: kvm@...r.kernel.org
>> > Cc: linux-doc@...r.kernel.org
>> > Cc: linux-kernel@...r.kernel.org
>> > Cc: Jan H. Schoenherr <jschoenh@...zon.de>
>> > Cc: Anthony Liguori <aliguori@...zon.com>
>> > Suggested-by: Matt Wilson <msw@...zon.com>
>> > Signed-off-by: Eduardo Valentin <eduval@...zon.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，I think it will result in "warning: host
doesn't support requested feature: CPUID.40000001H:eax.XXXXXX"

Regards,
Wanpeng Li