Re: [PATCH 4/4] Modify KVM to update guest time accounting.

Laurent Vivier
2007-08-20T14:22:05Z

[-- Attachment #1.1: Type: text/plain, Size: 2290 bytes --]

Avi Kivity wrote:
> Laurent Vivier wrote:
>> Avi Kivity wrote:
>>
>>> Laurent Vivier wrote:
>>>
>>>> [PATCH 4/4] Modify KVM to update guest time accounting.
>>>>
>>>>
>>>>
>>>> Index: kvm/drivers/kvm/kvm.h
>>>> ===================================================================
>>>> --- kvm.orig/drivers/kvm/kvm.h 2007-08-20 13:23:53.000000000 +0200
>>>> +++ kvm/drivers/kvm/kvm.h 2007-08-20 13:23:58.000000000 +0200
>>>> @@ -589,6 +589,17 @@ void kvm_mmu_unload(struct kvm_vcpu *vcp
>>>>
>>>> int kvm_hypercall(struct kvm_vcpu *vcpu, struct kvm_run *run);
>>>>
>>>> +static inline void kvm_guest_enter(void)
>>>> +{
>>>> +#ifdef CONFIG_GUEST_ACCOUNTING
>>>> + current->flags |= PF_VCPU;
>>>> +#endif
>>>>
>>>>
>>> Since you select GUEST_ACCOUNTING, the #ifdef is always true?
>>>
>> It is just to avoid error if we compile kvm module outside of the kernel, with
>> kernel without support of guest accounting. Can be removed if you want.
>>
>>
>
> Ah okay. We take care of the external module in
> external-module-compat.h (in this case, we can #define PF_VCPU to 0 if
> it is undefined).

OK

>>>> +}
>>>> +
>>>> +static inline void kvm_guest_exit(void)
>>>> +{
>>>>
>>>>
>>> Missing content?
>>>
>> No, empty function.
>>
>> It is just to introduce "symmetry" in code, the VCPU code begins with
>> "kvm_guest_start()" and ends with "kvm_guest_exit()". And we have no operation
>> to do in kvm_guest_exit() (the PF_VCPU is cleared by account_system_time()), so
>> "empty function", removed by the optimizer (I hope).
>>
>
> But what if account_system_time() is not called (because we exited
> before the timer interrupt hit)?

account_system_time() is called by the scheduler on tick. If we clear PF_VCPU in
kvm_guest_exit(), the flag is always cleared when account_system_time() is
executed, so we account "0", if we call account_system_time() directly from
kvm_guest_exit() we account "1", so we subtract 1 to system time, and we have
perhaps not used 1 tick in system: in my opinion this implies wrong results.

Laurent
--
------------- Laurent.Vivier-6ktuUTfB/bM@public.gmane.org --------------
"Software is hard" - Donald Knuth

[-- Attachment #1.2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 189 bytes --]

[-- Attachment #2: Type: text/plain, Size: 315 bytes --]

-------------------------------------------------------------------------
This SF.net email is sponsored by: Splunk Inc.
Still grepping through log files to find problems? Stop.
Now Search log events and configuration files using AJAX and a browser.
Download your FREE copy of Splunk now >> http://get.splunk.com/

[-- Attachment #3: Type: text/plain, Size: 186 bytes --]