linux-kernel - Re: [PATCH RFC 4/5] x86,fpu: lazily skip FPU restore when still loaded

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Message-ID: <1475526171.4622.9.camel@redhat.com>
Date:   Mon, 03 Oct 2016 16:22:51 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, tglx@...utronix.de, pbonzini@...hat.com,
        mingo@...hat.com, luto@...nel.org, hpa@...or.com, bp@...e.de
Subject: Re: [PATCH RFC 4/5] x86,fpu: lazily skip FPU restore when still
 loaded

On Mon, 2016-10-03 at 13:04 -0700, Dave Hansen wrote:
> On 10/01/2016 01:31 PM, riel@...hat.com wrote:
> > 
> >  /*
> > + * Check whether an FPU's register set is still loaded in the CPU.
> > + */
> > +static inline bool fpu_lazy_skip_restore(struct fpu *fpu)
> > +{
> > +	bool still_loaded = (fpu->fpstate_active &&
> > +			     fpu->last_cpu ==
> > raw_smp_processor_id() &&
> > +			     __this_cpu_read(fpu_fpregs_owner_ctx)
> > == fpu);
> > +
> > +	fpu->fpregs_active = still_loaded;
> > +	return still_loaded;
> > +}
> I wonder if we should call this something more along the lines of
> fpregs_activate_fast(), which returns if it managed to do the
> activation
> fast or not.  I _think_ that's more along the lines of what it is
> actually doing.  The fact that it can be lazy is really an
> implementation detail.
> 
> What are the preempt rules with this thing?  This needs to be called
> in
> preempt-disabled contexts, right?

Indeed, all the FPU context switching code needs 
to be called in preempt-disabled contexts.

You do not want to get preempted halfway through
saving or restoring floating point registers.

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