Re: [patch] faster vgetcpu using sidt

[dean gaudet <dean@arctic.org>]

On Sat, 6 Jan 2007, dean gaudet wrote:

> below is a patch which improves vgetcpu latency on all x86_64 
> implementations i've tested.
> 
> Nathan Laredo pointed out the sgdt/sidt/sldt instructions are 
> userland-accessible and we could use their limit fields to tuck away a few 
> bits of per-cpu information.
...

i got a hold of a p4 (model 4) and ran the timings there:

                        baseline        patched
                no cache        cache
k8 pre-revF        21             14      16
k8 revF            31             14      17
core2              38             12      17
p4                 49             24      37

not as good as i hoped... i'll have to put the cache back in just for the 
p4... so i'll respin my patch with the cache back in place.

another thought occured to me -- 64-bit processes can't actually use their 
LDT can they?  in that case i could probably use sldt (faster than sidt) 
for 64-bit procs and fallback to sidt for 32-bit emulation (which doesn't 
exist for this vsyscall yet anyhow).

let me know if you have any other feedback.

thanks
-dean
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