Re: PML (Page Modification Logging) design for Xen

[Kai Huang <kai.huang@linux.intel.com>]

On 02/12/2015 03:09 PM, Tian, Kevin wrote:
>> From: Kai Huang [mailto:kai.huang@linux.intel.com]
>> Sent: Thursday, February 12, 2015 2:57 PM
>>
>> On 02/12/2015 02:54 PM, Tian, Kevin wrote:
>>>> From: Kai Huang [mailto:kai.huang@linux.intel.com]
>>>> Sent: Thursday, February 12, 2015 10:39 AM
>>>>>> PML needs to be enabled (allocate PML buffer, initialize PML index,
>>>>>> PML base address, turn PML on VMCS, etc) for all vcpus of the domain,
>>>>>> as PML buffer and PML index are per-vcpu, but EPT table may be shared
>>>>>> by vcpus. Enabling PML on partial vcpus of the domain won't work. Also
>>>>>> PML will only be enabled for the domain when it is switched to dirty
>>>>>> logging mode, and it will be disabled when domain is switched back to
>>>>>> normal mode. As looks vcpu number won't be changed dynamically
>> during
>>>>>> guest is running (correct me if I am wrong here), so we don't have to
>>>>>> consider enabling PML for new created vcpu when guest is in dirty
>>>>>> logging mode.
>>>>> There are exactly d->max_vcpus worth of struct vcpus (and therefore
>>>>> VMCSes) for a domain after creation, and will exist for the lifetime of
>>>>> the domain.  There is no dynamic adjustment of numbers of vcpus during
>>>>> runtime.
>>>> Good to know.
>>> could we at least detect and warn vcpu changes when PML is enabled?
>>> dirty logging happens out of guest's knowledge and there could be the
>>> case where user right online/offline a vcpu within that window.
>> Why is the warning necessary? There's no harm leaving PML enabled when
>> vcpu becomes offline.
> what about online? you need enable PML for newly-online vcpu since
> meaningful work may be scheduled to it within logdirty window.
Do you mean vcpu number (those offline + those online) can be changed 
during guest's runtime, ex, a new vcpu is created and becomes online 
after PML is enabled for the domain? Otherwise, I don't see a problem.

As long as the total number of vcpus remains constant, it's not a 
problem, as we only enable PML after all vcpus are created (and it 
remains constant), and the vcpu status is irrelevant.

Thanks,
-Kai
>
>> Also we will not disable PML for that vcpu when it becomes offline, in
>> which case we don't need to re-enable PML, which can fail, when vcpu
>> becomes online again. It simplifies the logic.
> offline is not a problem
>
> Thanks
> Kevin
>