linux-kernel - Re: [PATCH 3/4] paravirt: add virt_spin

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Message-ID: <6dcf43c9-07a5-84dd-5a8e-ff7f7ba825dd@redhat.com>
Date:   Wed, 6 Sep 2017 09:11:40 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        virtualization@...ts.linux-foundation.org, jeremy@...p.org,
        chrisw@...s-sol.org, akataria@...are.com, rusty@...tcorp.com.au,
        boris.ostrovsky@...cle.com, hpa@...or.com, tglx@...utronix.de,
        mingo@...hat.com
Subject: Re: [PATCH 3/4] paravirt: add virt_spin_lock pvops function

On 09/06/2017 09:06 AM, Peter Zijlstra wrote:
> On Wed, Sep 06, 2017 at 08:44:09AM -0400, Waiman Long wrote:
>> On 09/06/2017 03:08 AM, Peter Zijlstra wrote:
>>> Guys, please trim email.
>>>
>>> On Tue, Sep 05, 2017 at 10:31:46AM -0400, Waiman Long wrote:
>>>> For clarification, I was actually asking if you consider just adding one
>>>> more jump label to skip it for Xen/KVM instead of making
>>>> virt_spin_lock() a pv-op.
>>> I don't understand. What performance are you worried about. Native will
>>> now do: "xor rax,rax; jnz some_cold_label" that's fairly trival code.
>> It is not native that I am talking about. I am worry about VM with
>> non-Xen/KVM hypervisor where virt_spin_lock() will actually be called.
>> Now that function will become a callee-saved function call instead of
>> being inlined into the native slowpath function.
> But only if we actually end up using the test-and-set thing, because if
> you have paravirt we end up using that.

I am talking about scenario like a kernel with paravirt spinlock running
in a VM managed by VMware, for example. We may not want to penalize them
while there are alternatives with less penalty.

Cheers,
Longman