Re: SVM: vmload/vmsave-free VM exits?

[Valentine Sinitsyn <valentine.sinitsyn@gmail.com>]

Hi Jan,

On 07.04.2015 10:43, Jan Kiszka wrote:
> On 2015-04-05 19:12, Valentine Sinitsyn wrote:
>> 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.
>
> It is, at least 160 cycles with hot caches on an AMD A6-5200 APU, more
> towards 600 if they are colder (added some usleep to each loop in the test).
Great, thanks. Could you post absolute numbers, i.e how long do A and B 
take on your CPU?

Valentine

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