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

On 02/12/2015 08:34 PM, Tim Deegan wrote:

Thanks for posting this design!

At 16:28 +0800 on 11 Feb (1423668493), Kai Huang wrote:

- 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.
Sounds good.  I agree that there's no point in making this a per-VM

- 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.

No - you really ought to handle enabling this for new VCPUs.  There
have been cases in the past where VMs are put into log-dirty mode
before their VCPUs are assigned, and there might be again.
"Assigned" here means created?

It ought to be easy to handle, though - just one more check and
function call on the vcpu setup path.
I think "check and function call" means check function call to enable PML on this vcpu? Then what if enabling PML for vcpu fails (possible as it needs to allocate 4K PML buffer)? It's better to choose to roll back to use write protection instead of indicating failure of creating the vcpu. But in this case there will be problem if the domain has already been in log dirty mode as we might already have EPT table setup with D-bit clear for logdirty range, which means we need to re-check the logdirty ranges and re-set EPT table to be read-only. Does this sound reasonable?

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.

IIUC, you are suggesting leaving superpages handled as they are now,
with read-only EPTEs, and only using PML for single-page mappings.
That seems good. :)

- 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.

I would prefer to flush only on buffer-full VMEXITs and handle the
peek/clear path by explicitly reading all VCPUs' buffers.  That avoids
putting more code on the fast paths for other VMEXIT types.
OK. But looks this requires a new interface like paging_flush_log_dirty, called at beginning of paging_log_dirty_op? This is actually what I wanted to avoid originally.

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.

- 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 

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 think, as other people have said, that you can just use PML for this
case without any other restrictions.  After all, mappings for non-VRAM
areas ought not to have their D-bits clear anyway.




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