linux-kernel - Re: [PATCH 06/18] arm64: move sve_user_{enable, disable} to <asm/fpsimd.h>

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Date:   Fri, 1 Jun 2018 11:29:13 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, linux@...inikbrodowski.net,
        james.morse@....com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 06/18] arm64: move sve_user_{enable, disable} to
 <asm/fpsimd.h>

On Wed, May 16, 2018 at 10:01:32AM +0100, Dave Martin wrote:
> On Tue, May 15, 2018 at 05:33:52PM +0100, Mark Rutland wrote:
> > On Tue, May 15, 2018 at 01:19:26PM +0100, Dave Martin wrote:
> > > On Tue, May 15, 2018 at 11:39:36AM +0100, Mark Rutland wrote:
> > > > Earlier I'd put BUILD_BUG() in the body for the !CONFIG_ARM64_SVE case,
> > > > to catch that kind of thing -- I could restore that.
> > > 
> > > IIUC:
> > > 
> > > 	if (0) {
> > > 		BUILD_BUG_ON(1);
> > > 	}
> > > 
> > > can still fire, in which case it's futile checking for CONFIG_ARM64_SVE
> > > in most of the SVE support code.
> > 
> > We already rely on BUILD_BUG() not firing in paths that can be trivially
> > optimized away. e.g. in the cmpxchg code.
> 
> Fair enough.  I had been unsure on this point.
> 
> If you want to put a BUILD_BUG_ON(!IS_ENABLED(CONFIG_ARM64_SVE)) in
> sve_user_enable() and build with CONFIG_ARM64_SVE=n to check it works,
> then I'd be fine with that.
> 
> This doesn't capture the runtime part of the condition, but it's better
> than nothing.

For the moment, I've kept the stubs, but placed a BUILD_BUG() in each,
as per the above rationale. 

We generally do that rather than than BUILD_BUG_ON(!IS_ENABLED(...)) in
a common definition, and it's more in keeping with the other stubs in
<asm/fpsimd.h>.

> > > > > > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > > > > > index 088940387a4d..79a81c7d85c6 100644
> > > > > > --- a/arch/arm64/kernel/fpsimd.c
> > > > > > +++ b/arch/arm64/kernel/fpsimd.c
> > > > > > @@ -159,7 +159,6 @@ static void sve_free(struct task_struct *task)
> > > > > >  	__sve_free(task);
> > > > > >  }
> > > > > >  
> > > > > > -
> > > > > 
> > > > > Hmmm, Ack.  Check for conflicts with the KVM FPSIMD rework [1] (though
> > > > > trivial).
> > > > 
> > > > I'll assume that Ack stands regardless. :)
> > > 
> > > Actually, I was just commenting on the deleted blank line... 
> > 
> > Ah. I've restored that now.
> 
> I meant Ack to the deletion.  It looks like the blank line was
> spuriously introduced in the first place.  But it doesn't hugely matter
> either way.

Ok. I've dropped that for now to minimize the potential for conflicts,
but we can clean this up later.

Thanks,
Mark.