Re: [PATCH v4] kvm: Use a bitmap for tracking used GSIs

[Alex Williamson <alex.williamson@hp.com>]

On Wed, 2009-05-13 at 16:55 +0300, Michael S. Tsirkin wrote:
> On Wed, May 13, 2009 at 07:11:16AM -0600, Alex Williamson wrote:
> > On Wed, 2009-05-13 at 16:00 +0300, Avi Kivity wrote:
> > > Alex Williamson wrote:
> > > >>
> > > >> When you say "multiple times", it is several, or a lot more?
> > > >>
> > > >> Maybe it is NAPI?
> > > >>     
> > > >
> > > > The system would run out of the ~1000 available GSIs in a minute or two
> > > > with just an e1000e available to the guest.  So that's something on the
> > > > order of 10/s.  This also causes a printk in the host ever time the
> > > > interrupt in enabled, which can't help performance and gets pretty
> > > > annoying for syslog.  I was guessing some kind of interrupt mitigation,
> > > > such as NAPI, but a qlogic FC card seems to do it too (seemingly at a
> > > > slower rate).
> > > >   
> > > 
> > > I see.  And what is the path by which it is disabled?  The mask bit in 
> > > the MSI entry?
> > 
> > Yes, I believe the only path is via a write to the MSI capability in the
> > PCI config space.
> > 
> > Alex
> 
> Very surprising: I haven't seen any driver disable MSI expect on device
> destructor path. Is this a linux guest?

Yes, Debian 2.6.26 kernel.  I'll check it it behaves the same on newer
upstream kernels and try to figure out why it's doing it.

Alex