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Re: [Xen-devel] PML (Page Modification Logging) design for Xen

On 02/11/2015 09:06 PM, Jan Beulich wrote:
On 11.02.15 at 09:28, <kai.huang@xxxxxxxxxxxxxxx> 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.



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