Re: [PATCH 1/1] x86/entry: Use one cmpq in NMI entry to check RIP for nested NMIs

[Petr Tesařík]

On Tue, 12 Mar 2024 16:03:17 +0100
Petr Tesarik <petrtesarik@xxxxxxxxxxxxxxx> wrote:

> On 2/29/2024 9:37 AM, Petr Tesarik wrote:
> > From: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> > 
> > Optimize the check whether a nested NMI occurred between repeat_nmi and
> > end_repeat_nmi. Although this is not a hot path, this is standard code to
> > check whether a value is within a given range; it is slightly faster, takes
> > up less bytes of code and saves one entry in the branch predictor.
> > 
> > This patch also removes the only relocation for end_repeat_nmi, removing
> > the need for ANNOTATE_NOENDBR.  
> 
> Any comment on this?

Ping. If you don't like the change, can I at least get an orderly NAK?

(Or have I made it onto an x86 maintainer blacklist?)

Petr T

> Kind regards
> Petr T
> 
> > Signed-off-by: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/entry/entry_64.S | 13 ++++---------
> >  1 file changed, 4 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index 9bb485977629..cae40076e109 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -1251,13 +1251,10 @@ SYM_CODE_START(asm_exc_nmi)
> >  	 * the outer NMI.
> >  	 */
> >  
> > -	movq	$repeat_nmi, %rdx
> > -	cmpq	8(%rsp), %rdx
> > -	ja	1f
> > -	movq	$end_repeat_nmi, %rdx
> > -	cmpq	8(%rsp), %rdx
> > -	ja	nested_nmi_out
> > -1:
> > +	movq	8(%rsp), %rdx
> > +	subq	$repeat_nmi, %rdx
> > +	cmpq	$(end_repeat_nmi - repeat_nmi), %rdx
> > +	jb	nested_nmi_out
> >  
> >  	/*
> >  	 * Now check "NMI executing".  If it's set, then we're nested.
> > @@ -1383,8 +1380,6 @@ repeat_nmi:
> >  	.endr
> >  	subq	$(5*8), %rsp
> >  end_repeat_nmi:
> > -	ANNOTATE_NOENDBR // this code
> > -
> >  	/*
> >  	 * Everything below this point can be preempted by a nested NMI.
> >  	 * If this happens, then the inner NMI will change the "iret"  
>