From: David Woodhouse <dwmw2@infradead.org>
To: Andi Kleen <andi@firstfloor.org>, tglx@linutronix.de
Cc: linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org,
torvalds@linux-foundation.org, arjan@linux.intel.com,
dave.hansen@intel.com, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] x86/retpoline: Optimize inline assembler for vmexit_fill_RSB
Date: Thu, 18 Jan 2018 13:41:09 +0100 [thread overview]
Message-ID: <1516279269.4937.125.camel@infradead.org> (raw)
In-Reply-To: <20180117225328.15414-1-andi@firstfloor.org>
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On Wed, 2018-01-17 at 14:53 -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.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@linux.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@amazon.co.uk>
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next prev parent reply other threads:[~2018-01-18 12:41 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-17 22:53 [PATCH] x86/retpoline: Optimize inline assembler for vmexit_fill_RSB Andi Kleen
2018-01-18 12:41 ` David Woodhouse [this message]
2018-01-19 15:49 ` [tip:x86/pti] " tip-bot for Andi Kleen
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