linux-kernel - Re: [PATCH] x86/retpoline: Optimize inline assembler for vmexit_fill

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Date:   Thu, 18 Jan 2018 13:41:09 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        torvalds@...ux-foundation.org, arjan@...ux.intel.com,
        dave.hansen@...el.com, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/retpoline: Optimize inline assembler for
 vmexit_fill_RSB

On Wed, 2018-01-17 at 14:53 -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> I was looking at the generated assembler for the C fill RSB
> inline asm operations, and noticed several issues:
> 
> - The C code sets up the loop register, which
> is then immediately overwritten in __FILL_RETURN_BUFFER
> with the same value again.
> 
> - The C code also passes in the iteration count
> in another register, which is not used at all.
> 
> Remove these two unnecessary operations. Just rely on
> the single constant passed to the macro for the iterations.
> 
> This eliminates several instructions and avoids unnecessarily
> clobbering a register.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

We still clobber the register, but you're right it's now filled in the
__FILL_RETURN_BUFFER macro itself. It was a previous iteration which
had the loop count passed in.

Acked-by: David Woodhouse <dwmw@...zon.co.uk>

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