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Re: Question Regarding ERMS memcpy

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On March 4, 2017 4:14:47 PM PST, Borislav Petkov <bp@suse.de> wrote:
>On Sat, Mar 04, 2017 at 03:55:27PM -0800, hpa@zytor.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.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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Question Regarding ERMS memcpy Logan Gunthorpe <logang@deltatee.com> - 2017-03-04 22:20 +0100
  Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-04 23:50 +0100
    Re: Question Regarding ERMS memcpy hpa@zytor.com - 2017-03-05 01:00 +0100
      Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 01:20 +0100
        Re: Question Regarding ERMS memcpy hpa@zytor.com - 2017-03-05 01:30 +0100
          Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 01:40 +0100
            Re: Question Regarding ERMS memcpy hpa@zytor.com - 2017-03-05 02:00 +0100
              Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 11:00 +0100
                Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 12:20 +0100
                Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 14:10 +0100
                Re: Question Regarding ERMS memcpy Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-05 20:30 +0100
                Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 21:20 +0100
                Re: Question Regarding ERMS memcpy Linus Torvalds <torvalds@linux-foundation.org> - 2017-03-05 21:20 +0100
                Re: Question Regarding ERMS memcpy Logan Gunthorpe <logang@deltatee.com> - 2017-03-06 08:10 +0100
                Re: Question Regarding ERMS memcpy "H. Peter Anvin" <hpa@zytor.com> - 2017-03-06 08:40 +0100
                Re: Question Regarding ERMS memcpy Logan Gunthorpe <logang@deltatee.com> - 2017-03-06 18:50 +0100
                Re: Question Regarding ERMS memcpy hpa@zytor.com - 2017-03-06 20:10 +0100
                Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-06 14:40 +0100
                Re: Question Regarding ERMS memcpy hpa@zytor.com - 2017-03-06 14:50 +0100
                Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-06 15:10 +0100
            Re: Question Regarding ERMS memcpy Logan Gunthorpe <logang@deltatee.com> - 2017-03-05 06:00 +0100
              Re: Question Regarding ERMS memcpy Borislav Petkov <bp@suse.de> - 2017-03-05 11:00 +0100

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