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x86/asm/entry/32: Stop caching MSR_IA32_SYSENTER_ESP in tss.sp1

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We write a stack pointer to MSR_IA32_SYSENTER_ESP exactly once,
and we unnecessarily cache the value in tss.sp1.  We never
read the cached value.

Remove all of the caching.  It serves no purpose.

Suggested-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/05a0163eb33ef5208363f0015496855da7cebadd.1428002830.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2015-04-02 12:41:45 -07:00 • committed by Ingo Molnar
parent ff8287f363
commit cf9328cc99
2 changed files with 16 additions and 15 deletions
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22
arch/x86/include/asm/processor.h
View file

@ -209,21 +209,21 @@ struct x86_hw_tss {
unsigned short back_link, __blh;
unsigned long sp0;
unsigned short ss0, __ss0h;
unsigned long sp1;
/*
* We don't use ring 1, so sp1 and ss1 are convenient scratch
* spaces in the same cacheline as sp0. We use them to cache
* some MSR values to avoid unnecessary wrmsr instructions.
* We don't use ring 1, so ss1 is a convenient scratch space in
* the same cacheline as sp0. We use ss1 to cache the value in
* MSR_IA32_SYSENTER_CS. When we context switch
* MSR_IA32_SYSENTER_CS, we first check if the new value being
* written matches ss1, and, if it's not, then we wrmsr the new
* value and update ss1.
*
* We use SYSENTER_ESP to find sp0 and for the NMI emergency
* stack, but we need to context switch it because we do
* horrible things to the kernel stack in vm86 mode.
*
* We use SYSENTER_CS to disable sysenter in vm86 mode to avoid
* corrupting the stack if we went through the sysenter path
* from vm86 mode.
* The only reason we context switch MSR_IA32_SYSENTER_CS is
* that we set it to zero in vm86 tasks to avoid corrupting the
* stack if we were to go through the sysenter path from vm86
* mode.
*/
unsigned long sp1; /* MSR_IA32_SYSENTER_ESP */
unsigned short ss1; /* MSR_IA32_SYSENTER_CS */
unsigned short __ss1h;

9
arch/x86/kernel/cpu/common.c
View file

@ -976,15 +976,16 @@ void enable_sep_cpu(void)
goto out;
/*
* The struct::SS1 and tss_struct::SP1 fields are not used by the hardware,
* we cache the SYSENTER CS and ESP values there for easy access:
* We cache MSR_IA32_SYSENTER_CS's value in the TSS's ss1 field --
* see the big comment in struct x86_hw_tss's definition.
*/
tss->x86_tss.ss1 = __KERNEL_CS;
wrmsr(MSR_IA32_SYSENTER_CS, tss->x86_tss.ss1, 0);
tss->x86_tss.sp1 = (unsigned long)tss + offsetofend(struct tss_struct, SYSENTER_stack);
wrmsr(MSR_IA32_SYSENTER_ESP, tss->x86_tss.sp1, 0);
wrmsr(MSR_IA32_SYSENTER_ESP,
(unsigned long)tss + offsetofend(struct tss_struct, SYSENTER_stack),
0);
wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long)ia32_sysenter_target, 0);