On 02/12/2018 02:36 PM, David Laight wrote:
From: Denys Vlasenko
Sent: 12 February 2018 13:29
x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro
Those instances where ALLOC_PT_GPREGS_ON_STACK is called just before
SAVE_AND_CLEAR_REGS can trivially be replaced by PUSH_AND_CLEAN_REGS.
This macro uses PUSH instead of MOV and should therefore be faster, at
least on newer CPUs.
Signed-off-by: Ingo Molnar <mi...@kernel.org>
arch/x86/entry/calling.h | 36 ++++++++++++++++++++++++++++++++++++
arch/x86/entry/entry_64.S | 6 ++----
2 files changed, 38 insertions(+), 4 deletions(-)
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index a05cbb8..57b1b87 100644
@@ -137,6 +137,42 @@ For 32-bit we have the following conventions - kernel is
+ .macro PUSH_AND_CLEAR_REGS
+ * Push registers and sanitize registers of values that a
+ * speculation attack might otherwise want to exploit. The
+ * lower registers are likely clobbered well before they
+ * could be put to use in a speculative execution gadget.
+ * Interleave XOR with PUSH for better uop scheduling:
+ pushq %rdi /* pt_regs->di */
+ pushq %rsi /* pt_regs->si */
+ pushq %rdx /* pt_regs->dx */
+ pushq %rcx /* pt_regs->cx */
+ pushq %rax /* pt_regs->ax */
+ pushq %r8 /* pt_regs->r8 */
+ xorq %r8, %r8 /* nospec r8 */
xorq's are slower than xorl's on Silvermont/Knights Landing.
I propose using xorl instead.
Does using movq to copy the first zero to the other registers make
the code any faster?
ISTR mov reg-reg is often implemented as a register rename rather than an
xorl is implemented in register rename as well. Just, for some reason,
xorq did not get the same treatment on those CPUs.