x86/entry/64: Correct the comment over SAVE_AND_SET_GSBASE

Add the proper explanation why an LFENCE is not needed in the FSGSBASE
case.

Fixes: c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200821090710.GE12181@zn.tnic

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 70dea93..bf78de4 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -840,8 +840,9 @@ SYM_CODE_START_LOCAL(paranoid_entry)
         * retrieve and set the current CPUs kernel GSBASE. The stored value
         * has to be restored in paranoid_exit unconditionally.
         *
-        * The MSR write ensures that no subsequent load is based on a
-        * mispredicted GSBASE. No extra FENCE required.
+        * The unconditional write to GS base below ensures that no subsequent
+        * loads based on a mispredicted GS base can happen, therefore no LFENCE
+        * is needed here.
         */
        SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
        ret