Re: [PATCH v2 4/4] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

[Jan Kiszka <jan.kiszka@web.de>]

[-- Attachment #1: Type: text/plain, Size: 1512 bytes --]

Am 02.11.2010 20:14, Michael S. Tsirkin wrote:
>>> BTW block userspace access uses a global spinlock which will likely hurt
>>> us on multi-CPU. Switching that to something more SMP friendly, e.g. a
>>> per-device spinlock, might be a good idea: I don't see why that lock and
>>> queue are global.
>>
>> Been through that code recently, hairy stuff. pci_lock also protects the
>> bus operation which can be overloaded (e.g. for error injection - if
>> that is not the only use case). So we need a per-bus lock, but that can
>> still cause contentions if devices on the same bus are handled on
>> different cpus.
> 
> Right. So this lock got reused for blocking userspace, I do not suggest
> we rip it all out, just make userspace blocking use
> a finer-grained lock.
> 
>> I think the whole PCI config interface is simply not designed for
>> performance. It's considered a slow path, which it normally is.
> 
> So I guess we'll need to fix that now, at least if we
> want to make the 2.3 way the default.
> 

On many systems (those with "direct" PCI config access), there is
another lock down the road: pci_config_lock. That can't be broken up as
the protected resource is unique. So we do not gain much improving the
higher-level lock.

BTW, accessing the interrupt controller for IRQ line fiddling is not a
per-device thing either. So as long as the latency of the code under the
locks is not significantly worse with PCI-level masking, we don't lose
scalability here.

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 259 bytes --]