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

[Jan Kiszka <jan.kiszka@web.de>]

On 2015-04-07 08:29, Valentine Sinitsyn wrote:
> On 07.04.2015 11:23, Jan Kiszka wrote:
>> On 2015-04-07 08:19, Valentine Sinitsyn wrote:
>>> On 07.04.2015 11:13, Jan Kiszka wrote:
>>>>>> 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?
>>>>
>>>> A is around 1910 cycles, B about 1750.
>>> It's with hot caches I guess? Not bad anyways, it's a pity I didn't
>>> observe this and didn't include this optimization from the day one.
>>
>> Yes, that is with the unmodified benchmark I sent. When I add, say
>> usleep(1000) to that loop body, the cycles jumped to 4k (IIRC).
>>
>> BTW, this is the Jailhouse patch:
>> https://github.com/siemens/jailhouse/commit/dbf2fe479ac07a677462dfa87e008e37a4e72858
>>
> I guess, it's getting off-topic here, but wouldn't it be cleaner to
> simply use wrmsr and rdmsr instead of vmload and vmsave in svm-vmexit.S?
> This would require less changes and will keep all entry/exit setup code
> in one place.

It's a tradeoff between assembly lines and C statements. My feeling is
that it's easier done in C, but you can prove me wrong.

Jan

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