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

Michael S. Tsirkin
2013-11-26T16:08:49Z

On Tue, Nov 26, 2013 at 06:06:26PM +0200, Avi Kivity wrote:
> On 11/26/2013 05:58 PM, Paolo Bonzini wrote:
> >Il 26/11/2013 16:35, Avi Kivity ha scritto:
> >>>>>If we want to ensure, we need to use a different mechanism for
> >>>>>synchronization than the global RCU. QRCU would work; readers are not
> >>>>>wait-free but only if there is a concurrent synchronize_qrcu, which
> >>>>>should be rare.
> >>>>An alternative path is to convince ourselves that the hardware does not
> >>>>provide the guarantees that the current code provides, and so we can
> >>>>relax them.
> >>>No, I think it's a reasonable guarantee to provide.
> >>Why?
> >Because IIUC the semantics may depend not just on the interrupt
> >controller, but also on the specific PCI device. It seems safer to
> >assume that at least one device/driver pair wants this to work.
>
> 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.

> The fact
> that Linux does interrupt migration from within the interrupt
> handler also shows that someone else believes that it is the only
> safe place to do it.
>