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

From: Jason Baron <jbaron@redhat.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	"David S. Miller" <davem@davemloft.net>,
	David Daney <david.daney@cavium.com>,
	Michael Ellerman <michael@ellerman.id.au>,
	Jan Glauber <jang@linux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Xen Devel <xen-devel@lists.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
	peterz@infradead.org, rth@redhat.com
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out
Date: Thu, 6 Oct 2011 14:33:19 -0400	[thread overview]
Message-ID: <20111006183319.GB2505@redhat.com> (raw)
In-Reply-To: <4E8DF02B.7010406@goop.org>

On Thu, Oct 06, 2011 at 11:15:07AM -0700, Jeremy Fitzhardinge wrote:
> On 10/06/2011 11:10 AM, Jason Baron wrote:
> > 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...
> 
> I'll have a look at it later today if I get a moment.  Should be fairly
> straightforward.
> 

cool. It does add some complication, I think...detecting the 2-byte vs.
5-byte, and if done at boot time, possibly taking the undesired
branch...

> What about the rest of the series.  Do you think it looks cooked enough
> for next mergewindow?
> 
>     J

Yes, it looks good to me thanks! Feel free to add my ack to the series.

thanks,

-Jason