On 02/12/2018 02:36 PM, David Laight wrote:
From: Denys VlasenkoSent: 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....Link: http://lkml.kernel.org/r/20180211104949.12992-5-linux@xxxxxxxxxxxxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- 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 --- a/arch/x86/entry/calling.h +++ b/arch/x86/entry/calling.h @@ -137,6 +137,42 @@ For 32-bit we have the following conventions - kernel is built with UNWIND_HINT_REGS offset=\offset .endm + .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 alu operation.
xorl is implemented in register rename as well. Just, for some reason, xorq did not get the same treatment on those CPUs. -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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