linux-kernel - Re: [PATCH v4 1/2] arm64: adjust el0_sync so that a function can be called

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Message-ID: <20140523144457.GN9252@arm.com>
Date:	Fri, 23 May 2014 15:44:57 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Larry Bassel <larry.bassel@...aro.org>
Cc:	Christopher Covington <cov@...eaurora.org>,
	Will Deacon <Will.Deacon@....com>,
	"khilman@...aro.org" <khilman@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 1/2] arm64: adjust el0_sync so that a function can be
 called

On Thu, May 22, 2014 at 11:35:20PM +0100, Larry Bassel wrote:
> > On 05/22/2014 03:27 PM, 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>
> > > ---
> > >  arch/arm64/kernel/entry.S | 24 +++++++++++++++++-------
> > >  1 file changed, 17 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > > index e8b23a3..20b336e 100644
> > > --- a/arch/arm64/kernel/entry.S
> > > +++ b/arch/arm64/kernel/entry.S
> > > @@ -354,7 +354,6 @@ el0_sync:
> > >  	lsr	x24, x25, #ESR_EL1_EC_SHIFT	// exception class
> > >  	cmp	x24, #ESR_EL1_EC_SVC64		// SVC in 64-bit state
> > >  	b.eq	el0_svc
> > > -	adr	lr, ret_to_user
> > >  	cmp	x24, #ESR_EL1_EC_DABT_EL0	// data abort in EL0
> > >  	b.eq	el0_da
> > >  	cmp	x24, #ESR_EL1_EC_IABT_EL0	// instruction abort in EL0
> > > @@ -383,7 +382,6 @@ el0_sync_compat:
> > >  	lsr	x24, x25, #ESR_EL1_EC_SHIFT	// exception class
> > >  	cmp	x24, #ESR_EL1_EC_SVC32		// SVC in 32-bit state
> > >  	b.eq	el0_svc_compat
> > > -	adr	lr, ret_to_user
> > >  	cmp	x24, #ESR_EL1_EC_DABT_EL0	// data abort in EL0
> > >  	b.eq	el0_da
> > >  	cmp	x24, #ESR_EL1_EC_IABT_EL0	// instruction abort in EL0
> > > @@ -426,22 +424,26 @@ el0_da:
> > >  	/*
> > >  	 * Data abort handling
> > >  	 */
> > > -	mrs	x0, far_el1
> > > -	bic	x0, x0, #(0xff << 56)
> > > +	mrs	x26, far_el1
> > >  	// enable interrupts before calling the main handler
> > >  	enable_dbg_and_irq
> > > +	mov	x0, x26
> > > +	bic	x0, x0, #(0xff << 56)
> > 
> > Nit: I believe you can bit clear with x26 as the source register and omit the
> > move instruction.
> 
> Is that really an improvement (assuming it works)? Are we saving
> any cycles here? If so, does it matter? It is easy to see what
> the move instruction is doing.

Even if it's not noticeable, I would still reduce the number of lines by
one. BIC with immediate is just an alias for AND and it supports
different source and destination.

-- 
Catalin
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