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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. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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