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x86/entry/64: Further improve paranoid_entry comments

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  16561f27f94e ("x86/entry: Add some paranoid entry/exit CR3 handling comments")

... added some comments.  This improves them a bit:

 - When I first read the new comments, it was unclear to me whether
   they were referring to the case where paranoid_entry interrupted
   other entry code or where paranoid_entry was itself interrupted.
   Clarify it.

 - Remove the EBX comment.  We no longer use EBX as a SWAPGS
   indicator.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/c47daa1888dc2298e7e1d3f82bd76b776ea33393.1539542111.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2018-10-14 11:38:18 -07:00 committed by Ingo Molnar
parent 04f4f954b6
commit ae852495be
1 changed files with 4 additions and 6 deletions
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10
arch/x86/entry/entry_64.S
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@ -1189,15 +1189,13 @@ ENTRY(paranoid_entry)
1:
/*
* Always stash CR3 in %r14. This value will be restored,
* verbatim, at exit. Needed if kernel is interrupted
* after switching to the user CR3 value but before
* returning to userspace.
* verbatim, at exit. Needed if paranoid_entry interrupted
* another entry that already switched to the user CR3 value
* but has not yet returned to userspace.
*
* This is also why CS (stashed in the "iret frame" by the
* hardware at entry) can not be used: this may be a return
* to kernel code, but with a user CR3 value. The %ebx flag
* for SWAPGS is also unusable for CR3 because there is a
* window with a user GS and a kernel CR3.
* to kernel code, but with a user CR3 value.
*/
SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14