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x86/cpu: Probe the behavior of nulling out a segment at boot time 

AMD and Intel do different things when writing zero to a segment
selector.  Since neither vendor documents the behavior well and it's
easy to test the behavior, try nulling fs to see what happens.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rudolf Marek <r.marek@assembler.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/61588ba0e0df35beafd363dc8b68a4c5878ef095.1460075211.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-04-07 17:31:46 -07:00 committed by Ingo Molnar
parent d47b50e7a1
commit 7a5d670487
2 changed files with 32 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/x86/include/asm/cpufeatures.h
View File

@ -294,6 +294,7 @@
#define X86_BUG_FXSAVE_LEAK X86_BUG(6) /* FXSAVE leaks FOP/FIP/FOP */ #define X86_BUG_FXSAVE_LEAK X86_BUG(6) /* FXSAVE leaks FOP/FIP/FOP */
#define X86_BUG_CLFLUSH_MONITOR X86_BUG(7) /* AAI65, CLFLUSH required before MONITOR */ #define X86_BUG_CLFLUSH_MONITOR X86_BUG(7) /* AAI65, CLFLUSH required before MONITOR */
#define X86_BUG_SYSRET_SS_ATTRS X86_BUG(8) /* SYSRET doesn't fix up SS attrs */ #define X86_BUG_SYSRET_SS_ATTRS X86_BUG(8) /* SYSRET doesn't fix up SS attrs */
#define X86_BUG_NULL_SEG X86_BUG(9) /* Nulling a selector preserves the base */
#ifdef CONFIG_X86_32 #ifdef CONFIG_X86_32
/* /*

31
arch/x86/kernel/cpu/common.c
View File

@ -889,6 +889,35 @@ static void detect_nopl(struct cpuinfo_x86 *c)
#endif #endif
} }
static void detect_null_seg_behavior(struct cpuinfo_x86 *c)
{
#ifdef CONFIG_X86_64
/*
* Empirically, writing zero to a segment selector on AMD does
* not clear the base, whereas writing zero to a segment
* selector on Intel does clear the base. Intel's behavior
* allows slightly faster context switches in the common case
* where GS is unused by the prev and next threads.
*
* Since neither vendor documents this anywhere that I can see,
* detect it directly instead of hardcoding the choice by
* vendor.
*
* I've designated AMD's behavior as the "bug" because it's
* counterintuitive and less friendly.
*/
unsigned long old_base, tmp;
rdmsrl(MSR_FS_BASE, old_base);
wrmsrl(MSR_FS_BASE, 1);
loadsegment(fs, 0);
rdmsrl(MSR_FS_BASE, tmp);
if (tmp != 0)
set_cpu_bug(c, X86_BUG_NULL_SEG);
wrmsrl(MSR_FS_BASE, old_base);
#endif
}
static void generic_identify(struct cpuinfo_x86 *c) static void generic_identify(struct cpuinfo_x86 *c)
{ {
c->extended_cpuid_level = 0; c->extended_cpuid_level = 0;
@ -921,6 +950,8 @@ static void generic_identify(struct cpuinfo_x86 *c)
get_model_name(c); /* Default name */ get_model_name(c); /* Default name */
detect_nopl(c); detect_nopl(c);
detect_null_seg_behavior(c);
} }
static void x86_init_cache_qos(struct cpuinfo_x86 *c) static void x86_init_cache_qos(struct cpuinfo_x86 *c)