linux-kernel - Re: Question Regarding ERMS memcpy

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Message-ID: <BF3D2B38-BC19-4B66-840E-6E9DF9C3E649@zytor.com>
Date:   Sat, 04 Mar 2017 16:23:17 -0800
From:   hpa@...or.com
To:     Borislav Petkov <bp@...e.de>
CC:     Logan Gunthorpe <logang@...tatee.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Question Regarding ERMS memcpy

On March 4, 2017 4:14:47 PM PST, Borislav Petkov <bp@...e.de> wrote:
>On Sat, Mar 04, 2017 at 03:55:27PM -0800, hpa@...or.com wrote:
>> For newer processors, as determined by -mtune=, it is actually the
>> best option for an arbitrary copy.
>
>So his doesn't have ERMS - it is a SNB - so if for SNB REP_GOOD is
>the best option for memcpy, then we should probably build with
>-fno-builtin-memcpy unconditionally. Otherwise gcc apparently inserts
>its own memcpy variant.
>
>And this is probably wrong because we do the detection at boot time and
>not at build time.
>
>For example here it generates REP; MOVSL for the call in
>drivers/firmware/dmi_scan.c::dmi_scan_machine() which looks wrong to
>me.
>Length is 32 so it could just as well do REP; MOVSQ.
>
>IOW, we could do something like this:
>
>---
>diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>index 2d449337a360..c1b68d147b8d 100644
>--- a/arch/x86/Makefile
>+++ b/arch/x86/Makefile
>@@ -142,10 +142,7 @@ ifdef CONFIG_X86_X32
> endif
> export CONFIG_X86_X32_ABI
> 
>-# Don't unroll struct assignments with kmemcheck enabled
>-ifeq ($(CONFIG_KMEMCHECK),y)
>-	KBUILD_CFLAGS += $(call cc-option,-fno-builtin-memcpy)
>-endif
>+KBUILD_CFLAGS += $(call cc-option,-fno-builtin-memcpy)
> 
> # Stackpointer is addressed different for 32 bit and 64 bit x86
> sp-$(CONFIG_X86_32) := esp

What are the compilation flags?  It may be that gcc still does TRT depending on this call site.  I'd check what gcc6 or 7 generates, though.
-- 
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