Re: [Qemu-devel] [PATCH 2/3] KVM: add kvm_arch_irqchip_add_msi_route

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2013-06-21 at 17:10 -0600, Alex Williamson wrote:

> MSI-X is rather ugly.  As you suggest, we trap accesses to the MSI-X
> table.  We don't know how many vectors the guest is going to use, so we
> incrementally add them by disabling and re-enabling with a new vector
> count.  The host decides what to put in the table, we don't care.  All
> interrupts bounce through the host and get to the guest via eventfd,
> either through qemu or directly through KVM irqfd.
> 
> If an in-use vector is modified, we write the new "MSI route" to KVM,
> the host doesn't need to care.  If a vector is masked, we free the host
> irq handler w/o modifying the vector configuration.  There's a comment
> in hw/misc/vfio.c that we could also just bounce masked vectors through
> qemu and let it drop it if we wanted to completely avoid toggling the
> host.  Linux currently does not have a usable interface for masking
> vectors at the device.  Thanks,

Right. It might also be handy to add what Willy's been talking about
for a while now, which is an interface to enable/setup individual MSI-X
at runtime.

Cheers,
Ben.