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

[Kai Huang <kai.huang@xxxxxxxxxxxxxxx>]

On 02/12/2015 08:34 PM, Tim Deegan wrote:
> Hi,
>
> Thanks for posting this design!
>
> At 16:28 +0800 on 11 Feb (1423668493), Kai Huang wrote:
> > 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.
> Sounds good.  I agree that there's no point in making this a per-VM
> feature.
>
> > - 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 
> > 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 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.
Agreed.

Thanks,
-Kai
> Cheers,
>
> Tim.
>
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