On Tue, Jul 21, 2015 at 02:50:25PM -0400, Jason Baron wrote:
> hmmm...so this is a case where need to the default the branch
> to the out-of-line branch at boot. That is, we can't just enable
> the out-of-line branch at boot time, b/c it might be too late at
> that point? IE native_sched_clock() gets called very early?
Well, even the layout is wrong here. The optimal thing would be to have:
NOP
rdtsc
unlikely:
/* read jiffies */
at build time. And then at boot time, patch in the JMP over the NOP on
!use_tsc boxes. And RDTSC works always, no matter how early.
I'm fairly sure we can do that now with alternatives instead of jump
labels.
The problem currently is that the 5-byte NOP gets patched in with a JMP
so we have an unconditional forwards JMP to the RDTSC.
Now I'd put my money on most arches handling NOPs better then
unconditional JMPs and this is a hot path...
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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