Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table

[Paolo Bonzini <pbonzini@redhat.com>]

Il 26/11/2013 17:21, Avi Kivity ha scritto:
>>> It's indeed safe, but I think there's a nice win to be had if we
>>> drop the assumption.
>> I'm not arguing with that, but a minor commoent below:
>>
>>>> (BTW, PCI memory writes are posted, but configuration writes are not).
>>> MSIs are configured via PCI memory writes.
>>>
>>> By itself, that doesn't buy us anything, since the guest could flush
>>> the write via a read.  But I think the fact that the interrupt
>>> messages themselves are posted proves that it is safe.
>> FYI, PCI read flushes the interrupt itself in, too.
> 
> I guess that kills the optimization then.  Maybe you can do qrcu,
> whatever that is.

It's "srcu" (a separate SRCU instance specific to the irq routing
table), which I managed to misspell twice.

Actually, it turns out that qrcu actually exists
(http://lwn.net/Articles/223752/) and has extremely fast grace periods,
but read_lock/read_unlock are also more expensive.  So it was probably
some kind of Freudian slip.

Paolo