Re: [Qemu-devel] Re: KVM call agenda for Feb 9

Anthony Liguori
2010-02-09T15:08:41Z

On 02/09/2010 08:38 AM, Avi Kivity wrote:
> On 02/09/2010 04:18 PM, Anthony Liguori wrote:
>> On 02/09/2010 02:52 AM, Jan Kiszka wrote:
>>> Alexander Graf wrote:
>>>> On 09.02.2010, at 07:56, Avi Kivity wrote:
>>>>> - rcuify/fine-grain qemu locks
>>>> And this should be done either way, but is probably not a
>>>> short-term goal.
>>>>
>>> Indeed. We won't get around this longterm as it is a scalability
>>> bottleneck and a killer for RT guest load. We can't push everything
>>> into
>>> the kernel. Qemu needs a smart plan how to gradually convert its CPU
>>> and
>>> device model to fine-grained locking.
>>
>> The VCPU loops should be easy to convert to lockless operation. It's
>> easier to do this upstream but that requires a functioning IO thread.
>>
>> For the table based MMIO and PIO dispatch, RCU would be a good
>> locking choice since these structures are rarely updated. The tricky
>> bit is that the APIC has to be converted over to lockless before any
>> other device can be converted because just about every device wants
>> to inject an interrupt.
>
> The problem is that all the internal APIs now have to be threaded.
> Looking at hpet as a simple example, we have qemu_irq_pulse() and
> qemu_mod_timer().

Yes. But just as you mention, we have to approach it on a
device-by-device basis and incrementally convert over devices.

> We also have some lock inversion since hpet calls the timer but the
> timers also call hpet.

Right. Making the dispatch loop in the IO thread thread safe won't be
so bad either :-)

> qemu_irq_pulse() feeds the various interrupt controllers; not a
> problem for kernel irqchip but nontrivial for qemu's ioapic and pic.
>
> I'm not saying we should push hpet into the kernel to save userspace
> coding effort; there should be an independent reason to do this. But
> I don't think threading qemu is going to be anything near easy.

It's certainly not easy but I don't think it's impossibly hard.

Regards,

Anthony Liguori