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x86/entry/32: Clarify register saving in __switch_to_asm()

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commit 6690e86be8 ("sched/x86: Save [ER]FLAGS on context switch")
re-introduced the flags saving on context switch to prevent AC leakage.

The pushf/popf instructions are right among the callee saved register
section, so the comment explaining the save/restore is not entirely
correct.

Add a seperate comment to pushf/popf explaining the reason.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Thomas Gleixner 2019-11-16 11:12:03 +01:00
parent e638ad0080
commit a3ba966066
1 changed files with 7 additions and 1 deletions
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arch/x86/entry/entry_32.S
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@ -718,6 +718,11 @@ ENTRY(__switch_to_asm)
pushl %ebx
pushl %edi
pushl %esi
/*
* Flags are saved to prevent AC leakage. This could go
* away if objtool would have 32bit support to verify
* the STAC/CLAC correctness.
*/
pushfl
/* switch stack */
@ -740,8 +745,9 @@ ENTRY(__switch_to_asm)
FILL_RETURN_BUFFER %ebx, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
#endif
/* restore callee-saved registers */
/* Restore flags or the incoming task to restore AC state. */
popfl
/* restore callee-saved registers */
popl %esi
popl %edi
popl %ebx