linux-kernel - Re: [PATCH v5 1/2] arm64: adjust el0_sync so that a function can be called

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Message-ID: <20140529175236.GA24270@arm.com>
Date:	Thu, 29 May 2014 18:52:36 +0100
From:	Will Deacon <will.deacon@....com>
To:	Larry Bassel <larry.bassel@...aro.org>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"khilman@...aro.org" <khilman@...aro.org>
Subject: Re: [PATCH v5 1/2] arm64: adjust el0_sync so that a function can be
 called

On Wed, May 28, 2014 at 08:35:51PM +0100, Larry Bassel wrote:
> On 28 May 14 12:27, Will Deacon wrote:
> > On Mon, May 26, 2014 at 07:56:12PM +0100, Larry Bassel wrote:
> > > To implement the context tracker properly on arm64,
> > > a function call needs to be made after debugging and
> > > interrupts are turned on, but before the lr is changed
> > > to point to ret_to_user(). If the function call
> > > is made after the lr is changed the function will not
> > > return to the correct place.
> > > 
> > > For similar reasons, defer the setting of x0 so that
> > > it doesn't need to be saved around the function call
> > > (save far_el1 in x26 temporarily instead).
> > > 
> > > Signed-off-by: Larry Bassel <larry.bassel@...aro.org>
> > 
> > [...]
> > 
> > 
> > Why have you added this mov instruction?
> 
> I believe (please correct me if I'm wrong) that it is necessary.
> Here is why:
> 
> > > @@ -476,23 +481,27 @@ el0_undef:
> > >  	// enable interrupts before calling the main handler
> > >  	enable_dbg_and_irq
> > >  	mov	x0, sp
> > > +	adr	lr, ret_to_user
> > >  	b	do_undefinstr
> > >  el0_dbg:
> > >  	/*
> > >  	 * Debug exception handling
> > >  	 */
> > >  	tbnz	x24, #0, el0_inv		// EL0 only
> > > -	mrs	x0, far_el1
> > > +	mrs	x26, far_el1
> 
> needed because do_debug_exception may clobber x0, so save far_el1
> in x26 (as other parts of this patch do)

Actually, do_debug_exception consumes the FAR as its first parameter, so you
don't need to put this in x26 afaict.

> > > +	mov	x0, x26
> 
> needed because far_el1 is expected to be in x0 here
> 
> > >  	mov	x1, x25
> > >  	mov	x2, sp
> > >  	bl	do_debug_exception
> > >  	enable_dbg
> 
> [call to ct_user_exit will go here in the next patch, this may re-clobber x0]
> 
> > > +	mov	x0, x26
> 
> needed because far_el1 is expected to be in x0 here

Is it? ret_to_user doesn't care. Does ct_user_exit use the FAR? I don't
think it does...

Will
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