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 > >
