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

[Denys Vlasenko]

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.
> ...
> > > 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.