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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1516279269.4937.125.camel@infradead.org>
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>

Download attachment "smime.p7s" of type "application/x-pkcs7-signature" (5213 bytes)