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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/02/15 08:28, Kai Huang wrote:
> Hi all,
>
> PML (Page Modification Logging) is a new feature on Intel's Boardwell
> server platfrom targeted to reduce overhead of dirty logging
> mechanism. Below is the design for Xen. Would you help to review and
> give comments?

Thankyou for this design.  It is a very good starting point!

>
> Background
> ==========
>
> Currently, dirty logging is done via write protection, which basically
> sets guest memory we want to log to be read-only, then when guest
> performs write to that memory, write fault (EPT violation in case of
> EPT is used) happens, in which we are able to log the dirty GFN. This
> mechanism works but at cost of one write fault for each write from the
> guest.

Strictly speaking, repeated writes to the same gfn after the first fault
are amortised until the logdirty is next queried, which makes typical
access patterns far less costly than a fault for every single write.

>
> PML Introduction
> ================
>
> PML is a hardware-assisted efficient way, based on EPT mechanism, for
> dirty logging. Briefly, PML logs dirty GPA automatically to a 4K PML
> buffer when CPU changes EPT table's D-bit from 0 to 1. To accomplish
> this, A new PML buffer base address (machine address), a PML index,
> and a new PML buffer full VMEXIT were added to VMCS. Initially PML
> index can be set to 511 (8 bytes for each GPA) to indicate the buffer
> is empty, and CPU decreases PML index by 1 after logging GPA. Before
> performing GPA logging, PML checks PML index to see if PML buffer has
> been fully logged, in which case a PML buffer full VMEXIT happens, and
> VMM should flush logged GPAs (to data structure keeps dirty GPAs) and
> reset PML index so that further GPAs can be logged again.
>
> The specification of PML can be found at:
> http://www.intel.com/content/www/us/en/processors/page-modification-logging-vmm-white-paper.html
>
>
> With PML, we don't have to use write protection but just clear D-bit
> of EPT entry of guest memory to do dirty logging, with an additional
> PML buffer full VMEXIT for 512 dirty GPAs. Theoretically, this can
> reduce hypervisor overhead when guest is in dirty logging mode, and
> therefore more CPU cycles can be allocated to guest, so it's expected
> benchmarks in guest will have better performance comparing to non-PML.

One issue with basic EPT A/D tracking was the scan of the EPT tables. 
Here, hardware will give us a list of affected gfns, but how is Xen
supposed to efficiently clear the dirty bits again?  Using EPT
misconfiguration is no better than the existing fault path.

>
> Design
> ======
>
> - PML feature is used globally
>
> A new Xen boot parameter, say 'opt_enable_pml', will be introduced to
> control PML feature detection, and PML feature will only be detected
> if opt_enable_pml = 1. Once PML feature is detected, it will be used
> for dirty logging for all domains globally. Currently we don't support
> to use PML on basis of per-domain as it will require additional
> control from XL tool.

Rather than adding in a new top level command line option for an ept
subfeature, it would be preferable to add an "ept=" option which has
"pml" as a sub boolean.

>
> - PML enable/disable for particular Domain

I do not believe that this is an interesting use case at the moment. 
Currently, PML would be an implementation detail of how Xen manages to
provide the logdirty bitmap to the toolstack or device model, and need
not be exposed at all.

If in the future, a toolstack component wishes to use the pml for other
purposes, there is more infrastructure which needs adjusting than just
per-domain PML.

>
> 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.

>
> 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.

How is a superpage write reflected in the PML?

According to the whitepaper, transitioning the D bit from 0 to 1 results
in an entry being written into the log.  I presume that in the case of a
superpage, 512 entries are not written to the log, which presumably
means that the PML buffer flush needs to be aware of which gfns are
mapped by superpages to be able to correctly set a block of bits in the
logdirty bitmap.

>
> - 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.

Why apparently?  It would be quite easy for a guest to dirty 512 frames
without otherwise taking a vmexit.

>
> 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.

My gut feeling is that this is substantial overhead on a common path,
but this largely depends on how the dirty bits can be cleared efficiently.

>
> - Video RAM tracking (and partial dirty logging for guest memory range)
>
> Video RAM is in dirty logging mode unconditionally during guest's
> run-time, and it is partial memory range of the guest. However, PML
> operates on the whole guest memory (the whole valid EPT table, more
> precisely), so we need to choose whether to use PML if only partial
> guest memory ranges are in dirty logging mode.
>
> Currently, PML will be used as long as there's guest memory in dirty
> logging mode, no matter globally or partially. And in case of partial
> dirty logging, we need to check if the logged GPA in PML buffer is in
> dirty logging range.

I am not sure this is a problem.  HAP vram tracking already leaks
non-vram frames into the dirty bitmap, caused by calls to
paging_mark_dirty() from paths which are not caused by a p2m_logdirty fault.

~Andrew


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