Re: PML (Page Modification Logging) design for Xen

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

On 02/11/2015 09:06 PM, Jan Beulich wrote:
>>>> On 11.02.15 at 09:28, <kai.huang@linux.intel.com> wrote:
>> - PML enable/disable for particular Domain
>>
>> 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.
>>
>> After PML is enabled for the domain, we only need to clear EPT entry's D-bit
>> for guest memory in dirty logging mode. We achieve this by checking if PML is
>> enabled for the domain when p2m_ram_rx changed to p2m_ram_logdirty, and
>> updating EPT entry accordingly. However, for super pages, we still write
>> protect them in case of PML as we still need to split super page to 4K page
>> in dirty logging mode.
> While it doesn't matter much for our immediate needs, the
> documentation isn't really clear about the behavior when a 2M or
> 1G page gets its D bit set: Wouldn't it be rather useful to the
> consumer to know of that fact (e.g. by setting some of the lower
> bits of the PML entry to indicate so)?
This is good point. The documentation only tells us the GPA will be 
logged with last 12 bits cleared. Whether hardware just clears last 12 
bits or performs 2M alignment (in case of 2M page) is not certain. I 
will confirm this with hardware guys. But as you said, it's not related 
to our immediate needs.
>
>> - PML buffer flush
>>
>> There are two places we need to flush PML buffer. The first place is PML
>> buffer full VMEXIT handler (apparently), and the second place is in
>> paging_log_dirty_op (either peek or clean), as vcpus are running
>> asynchronously along with paging_log_dirty_op is called from userspace via
>> hypercall, and it's possible there are dirty GPAs logged in vcpus' PML
>> buffers but not full. Therefore we'd better to flush all vcpus' PML buffers
>> before reporting dirty GPAs to userspace.
>>
>> We handle above two cases by flushing PML buffer at the beginning of all
>> VMEXITs. This solves the first case above, and it also solves the second
>> case, as prior to paging_log_dirty_op, domain_pause is called, which kicks
>> vcpus (that are in guest mode) out of guest mode via sending IPI, which cause
>> VMEXIT, to them.
>>
>> This also makes log-dirty radix tree more updated as PML buffer is flushed
>> on basis of all VMEXITs but not only PML buffer full VMEXIT.
> Is that really efficient? Flushing the buffer only as needed doesn't
> seem to be a major problem (apart from the usual preemption issue
> when dealing with guests with very many vCPU-s, but you certainly
> recall that at this point HVM is still limited to 128).
>
> Apart from these two remarks, the design looks okay to me.
While keeping log-dirty radix tree more updated is probably irrelevant, 
I do think we'd better to flush PML buffers in paging_log_dirty_op (both 
peek and clear) before reporting dirty pages to userspace, in which case 
I think flushing PML buffer at beginning of VMEXIT is a good idea, as 
domain_pause does the job automatically. I am not sure how much cycles 
will flushing PML buffer contribute but I think it should be relatively 
small comparing to VMEXIT itself, therefore it can be ignored.

An optimized way probably is we only flush PML buffer for external 
interrupt VMEXIT, which domain_pause really triggers, but not at 
beginning of all VMEXITs. But as log as the overhead of flush PML buffer 
is negligible, this optimization is also unnecessary.

Thanks,
-Kai
>
> Jan
>
>
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