On 17.05.2009, at 23:08, Avi Kivity wrote:
Alexander Graf wrote:
In order to find out why things were slow with nested SVM I hacked
intercept reporting into debugfs in my local tree and found pretty
interesting results (using NPT):
[...]
So apparently the most intercepts come from the SVM helper calls
(clgi, stgi, vmload, vmsave). I guess I need to get back to the
"emulate when GIF=0" approach to get things fast.
There's only a limited potential here (a factor of three, reducing 6
exits to 2, less the emulation overhead). There's a lot more to be
gained from nested npt, since you'll avoid most of the original
exits in the first place.
I think the reversed is the case. Look at those numbers (w2k8 bootup):
http://pastebin.ca/1423596
The only thing nested NPT would achieve is a reduction of #NPF exits.
But they are absolutely in the minority today already. Normal #PF's do
get directly passed to the guest already.
Of course, this all depends on the workload. For kernbench style
benchmarks nested NPT probably gives you a bigger win, but anything
doing IO is slowed down way more than it has to now.
Alex
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