Hi Jan,
On 05.04.2015 13:31, Jan Kiszka wrote:
studying the VM exit logic of Jailhouse, I was wondering when AMD's
vmload/vmsave can be avoided. Jailhouse as well as KVM currently use
these instructions unconditionally. However, I think both only need
GS.base, i.e. the per-cpu base address, to be saved and restored if no
user space exit or no CPU migration is involved (both is always true for
Jailhouse). Xen avoids vmload/vmsave on lightweight exits but it also
still uses rsp-based per-cpu variables.
So the question boils down to what is generally faster:
A) vmload
vmrun
vmsave
B) wrmsrl(MSR_GS_BASE, guest_gs_base)
vmrun
rdmsrl(MSR_GS_BASE, guest_gs_base)
Of course, KVM also has to take into account that heavyweight exits
still require vmload/vmsave, thus become more expensive with B) due to
the additional MSR accesses.
Any thoughts or results of previous experiments?
That's a good question, I also thought about it when I was finalizing
Jailhouse AMD port. I tried "lightweight exits" with apic-demo but it
didn't seem to affect the latency in any noticeable way. That's why I
decided not to push the patch (in fact, I was even unable to find it now).
Note however that how AMD chips store host state during VM switches are
implementation-specific. I did my quick experiments on one CPU only, so
your mileage may vary.
Regarding your question, I feel B will be faster anyways but again I'm
afraid that the gain could be within statistical error of the experiment.
Valentine
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