Re: [Qemu-devel] Re: [PATCH 3/3] Add KVM support to QEMU

Glauber Costa
2008-10-29T12:33:47Z

On Wed, Oct 29, 2008 at 11:54:11AM +0200, Avi Kivity wrote:
> Anthony Liguori wrote:
>> Another place "hook" is updating a slot's dirty bitmap. Right now,
>> with my patchset we don't have live migration or the VGA RAM
>> optimization. There's nothing about the VGA RAM optimization that
>> wouldn't work for QEMU. I'm not sure that it really is an
>> optimization in the context of TCG, but I certainly don't think it's
>> any worse. The only thing you really need is to query the KVM dirty
>> bitmap when it comes time to enable start over querying the VGA dirty
>> bits.
>
> I don't understand this. The VGA optimization really is qemu's, the kvm
> modifications only cater to the different way of getting the dirty bits.

As it seems to me, the real difference is that qemu has to explicitly set
certain regions as dirty, while kvm get dirty bit "automatically" from the kernel.

So I believe we can have markers on the code to refresh dirty bitmap for certain
area ranges (for kvm use), and also enable a manual override (for qemu). After that,
the cpu_physical_memory_get_dirty() will simply return whether or not the page is
dirty.

Also, kvm only tracks "dirty" bits, whereas qemu has at least three kinds of them.
But I think for now we can assume that kvm's dirty mean "all dirty"