WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out
From: Jason Baron <jbaron@xxxxxxxxxx>
Date: Thu, 6 Oct 2011 14:10:55 -0400
Cc: the arch/x86 maintainers <x86@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, David Daney <david.daney@xxxxxxxxxx>, peterz@xxxxxxxxxxxxx, Jan Glauber <jang@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, rth@xxxxxxxxxx, Michael Ellerman <michael@xxxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxxxxx>
Delivery-date: Sun, 09 Oct 2011 09:22:16 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <4E8DEB19.1050509@xxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <cover.1317506051.git.jeremy.fitzhardinge@xxxxxxxxxx> <477dead9647029012f93c651f2892ed0e86b89e7.1317506051.git.jeremy.fitzhardinge@xxxxxxxxxx> <20111003150205.GB2462@xxxxxxxxxx> <4E89E28C.7010700@xxxxxxxx> <20111004141011.GA2520@xxxxxxxxxx> <4E8B3489.60902@xxxxxxxxx> <4E8CF348.4080405@xxxxxxxx> <4E8CF385.2080804@xxxxxxxxx> <4E8DEB19.1050509@xxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.20 (2009-12-10)
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;
> }


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>