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

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Date:   Tue, 5 Sep 2017 16:08:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 Tue, Sep 05, 2017 at 10:02:57AM -0400, Waiman Long wrote:
> On 09/05/2017 09:24 AM, Juergen Gross wrote:

> > +static inline bool native_virt_spin_lock(struct qspinlock *lock)
> > +{
> > +	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> > +		return false;
> > +
> 
> I think you can take the above if statement out as you has done test in
> native_pv_lock_init(). So the test will also be false here.

That does mean we'll run a test-and-set spinlock until paravirt patching
happens though. I prefer to not do that.

One important point.. we must not be holding any locks when we switch
over between the two locks. Back then I spend some time making sure that
didn't happen with the X86 feature flag muck.