x86/entry/64: Use JMP instead of JMPQ [Linux 4.4.189]

This Linux kernel change "x86/entry/64: Use JMP instead of JMPQ" is included in the Linux 4.4.189 release. This change is authored by Josh Poimboeuf <jpoimboe [at] redhat.com> on Mon Jul 15 11:51:39 2019 -0500. The commit for this change in Linux stable tree is 967858f (patch) which is from upstream commit 64dbc12. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream 64dbc12.

x86/entry/64: Use JMP instead of JMPQ

commit 64dbc122b20f75183d8822618c24f85144a5a94d upstream.

Somehow the swapgs mitigation entry code patch ended up with a JMPQ
instruction instead of JMP, where only the short jump is needed.  Some
assembler versions apparently fail to optimize JMPQ into a two-byte JMP
when possible, instead always using a 7-byte JMP with relocation.  For
some reason that makes the entry code explode with a #GP during boot.

Change it back to "JMP" as originally intended.

Fixes: 18ec54fdd6d1 ("x86/speculation: Prepare entry code for Spectre v1 swapgs mitigations")
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/entry/entry_64.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 0baac4a..afb805b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -567,7 +567,7 @@ END(irq_entries_start)
    call enter_from_user_mode
-   jmpq    2f
+   jmp 2f

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