RE: [tip:x86/pti] x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro

[David Laight]

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@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.

	David