Re: [Qemu-devel] removing on-demand msix vector allocation

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Dec 07, 2012 at 08:37:22AM +0100, Jan Kiszka wrote:
> On 2012-12-06 08:59, Michael S. Tsirkin wrote:
> > I've been looking at handling of msix masking
> > in qemu. It looks like all of virtio,vfio and
> > device assignment implemented their own
> > similar but slightly different thing.
> > So I am inclined to move this handling to common
> > code in msix.c, adding irqfd support right there.
> > 
> > While doing this rework, one of the more painful
> > bits of code to change is the code that dynamically
> > allocates msix table entries as we inject msi.
> > If this actually triggers it's going to be
> > painfully slow as route changes are rcu
> > write side in kernel.
> > Since recent kernels support direct injection,
> > do we care anymore? I think if you run out of
> > vectors, it's better to simply disable irqchip
> > than try to limp along changing routes all the time.
> 
> But how would the logic without dynamic allocation look like? Always
> configure a route in the PCI layer if an MSI/MSI-X entry is enabled?
> That would also affect emulated devices that don't use irqfd, thus you
> would waste routing entries.

Yes. 
So we can fail during initialization and ask user to
disable irqchip: at the moment, at least in my testing,
dynamic swap out of MSI entries performs very badly
anyway.

> OTOH, if don't set up such routes, you
> cannot support MSI/-X on older x86 kernels without direct vector injection.
> 
> Jan
> 
>