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

Anthony Liguori
2008-10-29T13:32:09Z

Avi Kivity wrote:
> Anthony Liguori wrote:
>> 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.
>>
>> Right. I'm just not sure that it's going to be as much of an
>> optimization for TCG as it is for KVM.
>
> When qemu is loaded certainly this is lost in the noise. But when
> idling (and as a bonus, the screen doesn't change much), tcg and kvm
> will benefit equally.
>
>>
>> Right now, in QEMU, code looks like this:
>>
>> for (i = 0; i < addr; i += TARGET_PAGE_SIZE) {
>> if (cpu_p_m_g_dirty(i, DIRTY_FLAG)) {
>> cpu_p_m_r_dirty(i, i + TARGET_PAGE_SIZE, DIRTY_FLAG);
>>
>> // do something with dirty memory
>> }
>>
>> All we need to do is add another cpu_physical_memory_sync_dirty(i, i
>> + REGION_SIZE, DIRTY_FLAG); that would go at the start of this. For
>> QEMU, this is a nop since dirty bits are updated as soon as they are
>> reset. For KVM, this would update the entire set of dirty bits for
>> the given memory region.
>
> You mean merge the kvm bitmap into the qemu bitmap? That's what it
> does now, no?
>
>> We also need something to enable dirty tracking for a particular
>> region. We already have something for migration, we could perhaps
>> extend that API (cpu_p_m_s_dirty_tracking).
>
> I don't think qemu would benefit much from per-region tracking, but
> maybe I'm wrong.

QEMU doesn't do per region tracking but instead, uses flags to simulate
regions. At any rate, the suggestion for an API to enable dirty
tracking was really for KVMs benefit. It's a nop for QEMU.

Regards,

Anthony Liguori