[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

[Jason Baron <jbaron@xxxxxxxxxx>]

On Thu, Oct 06, 2011 at 10:53:29AM -0700, Jeremy Fitzhardinge wrote:
> On 10/05/2011 05:17 PM, H. Peter Anvin wrote:
> > On 10/05/2011 05:16 PM, Jeremy Fitzhardinge wrote:
> >> On 10/04/2011 09:30 AM, H. Peter Anvin wrote:
> >>> On 10/04/2011 07:10 AM, Jason Baron wrote:
> >>>> 1) The jmp +0, is a 'safe' no-op that I know is going to initially
> >>>> boot for all x86. I'm not sure if there is a 5-byte nop that works on
> >>>> all x86 variants - but by using jmp +0, we make it much easier to debug
> >>>> cases where we may be using broken no-ops.
> >>>>
> >>> There are *plenty*.  jmp+0 is about as pessimal as you can get.
> >> As an aside, do you know if a 2-byte unconditional jmp is any more
> >> efficient than 5-byte, aside from just being a smaller instruction and
> >> taking less icache?
> >>
> > I don't know for sure, no.  I probably depends on the CPU.
> 
> Looks like jmp2 is about 5% faster than jmp5 on Sandybridge with this
> benchmark.
> 
> But insignificant difference on Nehalem.
> 
>     J

It would be cool if we could make the total width 2-bytes, when
possible.  It might be possible by making the initial 'JUMP_LABEL_INITIAL_NOP'
as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
time or link time - letting the compiler pick the width. In that way we could
get the optimal width...

thanks,

-Jason


> #include <stdio.h>
> 
> struct {
>       unsigned char flag;
>       unsigned char val;
> } l;
> 
> #define JMP2  asm volatile ("jmp 1f; .byte 0x0f,0x1f,0x00; 1: ");
> #define JMPJMP2       JMP2 JMP2
> #define JMPJMPJMP2    JMPJMP2 JMPJMP2
> #define JMPJMPJMP2    JMPJMP2 JMPJMP2
> #define JMPJMPJMPJMP2 JMPJMPJMP2 JMPJMPJMP2
> #define JMPJMPJMPJMPJMP2      JMPJMPJMPJMP2 JMPJMPJMPJMP2
> #define JMPJMPJMPJMPJMPJMP2   JMPJMPJMPJMPJMP2 JMPJMPJMPJMPJMP2
> 
> int main(int argc, char **argv)
> {
>       int i;
> 
>       for (i = 0; i < 100000000; i++) {
>               JMPJMPJMPJMPJMPJMP2;
>               asm volatile("" : : : "memory");
>       }
> 
>       return 0;
> }

> #include <stdio.h>
> 
> struct {
>       unsigned char flag;
>       unsigned char val;
> } l;
> 
> #define JMP5  asm volatile (".byte 0xe9; .long 0");
> #define JMPJMP5       JMP5 JMP5
> #define JMPJMPJMP5    JMPJMP5 JMPJMP5
> #define JMPJMPJMP5    JMPJMP5 JMPJMP5
> #define JMPJMPJMPJMP5 JMPJMPJMP5 JMPJMPJMP5
> #define JMPJMPJMPJMPJMP5      JMPJMPJMPJMP5 JMPJMPJMPJMP5
> #define JMPJMPJMPJMPJMPJMP5   JMPJMPJMPJMPJMP5 JMPJMPJMPJMPJMP5
> 
> int main(int argc, char **argv)
> {
>       int i;
> 
>       for (i = 0; i < 100000000; i++) {
>               JMPJMPJMPJMPJMPJMP5;
>               asm volatile("" : : : "memory");
>       }
> 
>       return 0;
> }


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