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SubjectRE: [tip:x86/pti] x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro
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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.
...
> > Link: http://lkml.kernel.org/r/20180211104949.12992-5-linux@dominikbrodowski.net
> > Signed-off-by: Ingo Molnar <mingo@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
> > --- 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.

David

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