Re: dirty page tracking in kvm/qemu -- page faults inevitable?

[Chris Friesen <cbf123@mail.usask.ca>]

On 07/30/2014 12:09 AM, Xiao Guangrong wrote:
> On 07/30/2014 06:12 AM, Chris Friesen wrote:
>> Hi,
>>
>> I've got an issue where we're hitting major performance penalties while doing live migration, and it seems like it might
>> be due to page faults triggering hypervisor exits, and then we get stuck waiting for the iothread lock which is held by
>> the qemu dirty page scanning code.
>
> I am afraid that using dirty-bit instead of write-protection may cause the case
> even more worse for iothread-lock because we need to walk whole sptes to get
> dirty-set pages, however currently we only need to walk the page set in the
> bitmap.

I found a document at
"http://ftp.software-sources.co.il/Processor_Architecture_Update-Bob_Valentine.pdf" 
which talks about the benefits of Haswell.  One of the items reads:

"New Accessed and Dirty bits for Extended Page Tables (EPT) eliminates 
major cause of vmexits"

Is that accurate?  If so, then it seems like it should allow for the VM 
to run without trying to exit the hypervisor, and as long as it just 
does in-memory operations it won't contend on the iothread lock.

Chris