x86/asm/entry/64: Simplify optimistic SYSRET

Avoid redundant load of %r11 (it is already loaded a few
instructions before).

Also simplify %rsp restoration, instead of two steps:

         add $0x80, %rsp
         mov 0x18(%rsp), %rsp

we can do a simplified single step to restore user-space RSP:

         mov 0x98(%rsp), %rsp

and get the same result.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
[ Clarified the changelog. ]
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1aef69b346a6db0d99cdfb0f5ba83e8c985e27d7.1424989793.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 6 insertions, 3 deletions

diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
index 337423590b08..f1a962ff7ddf 100644
--- a/arch/x86/include/asm/calling.h
+++ b/arch/x86/include/asm/calling.h
@@ -176,6 +176,9 @@ For 32-bit we have the following conventions - kernel is built with
         .macro RESTORE_C_REGS_EXCEPT_RCX
         RESTORE_C_REGS_HELPER 1,0,1,1,1
         .endm
+        .macro RESTORE_C_REGS_EXCEPT_R11
+        RESTORE_C_REGS_HELPER 1,1,0,1,1
+        .endm
         .macro RESTORE_RSI_RDI
         RESTORE_C_REGS_HELPER 0,0,0,0,0
         .endm
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 858e94e86f5e..bc1527889c40 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -757,9 +757,9 @@ retint_swapgs:		/* return to user-space */
          */
 irq_return_via_sysret:
         CFI_REMEMBER_STATE
-        RESTORE_C_REGS
-        REMOVE_PT_GPREGS_FROM_STACK 8
-        movq (RSP-RIP)(%rsp),%rsp
+        /* r11 is already restored (see code above) */
+        RESTORE_C_REGS_EXCEPT_R11
+        movq RSP(%rsp),%rsp
         USERGS_SYSRET64
         CFI_RESTORE_STATE