Re: [Qemu-devel] [PATCH] kvm: Move kvm_allows_irq0_override() to target-i386

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 23 Jul 2012 16:14:02 +0300
Avi Kivity <avi@redhat.com> wrote:

> On 07/23/2012 04:06 PM, Cornelia Huck wrote:
> > On Mon, 23 Jul 2012 15:18:49 +0300
> > Avi Kivity <avi@redhat.com> wrote:
> > 
> >> > So, for example, if a specific subchannel (=device) has pending status
> >> > and an I/O interrupt is to be generated, this interrupt remains pending
> >> > until an arbitrary cpu is enabled for I/O interrupts. If several cpus
> >> > are enabled for I/O interrupts, any of them may be interrupted.
> >> 
> >> This may be costly to emulate.  On x86 we do not have access to a
> >> guest's interrupt status while it is running.  Is this not the case for
> >> s390?
> >> 
> >> Oh, let me guess.  You write some interrupt descriptor in memory
> >> somewhere, issue one of your famous instructions, and the hardware finds
> >> a guest vcpu and injects the interrupt.
> > 
> > Basically, we have some flags in our control block we can set so that
> > the cpu drops out of SIE whenever external/I/O/... interrupts are
> > enabled and then have the host do the lowcore updates, psw swaps, etc.
> 
> Can you write them from a different cpu and expect them to take effect?
> 
> How do you emulate an interrupt with a large guest?  You have to update
> the flags in the control blocks for all vcpus; then restore them when
> the interrupt is delivered?

We may access the flags for any vcpu via the kvm_s390_float_interrupt
structure which is contained in kvm->arch.

We'll get control when a vcpu enters a wait state and try to deliver
pending interrupts or set/clear the flags. Moreover, idle vcpus are on
a wait queue and are the first target for getting an interrupt injected.

> >>  While you don't have an irqchip, you do have asynchronous interrupt
> >> injection, yes?  That's what irqchip really is all about.
> > 
> > You mean injection via ioctl() that is asynchronous to vcpu execution?
> > Yes, although we use a different ioctl than the others.
> 
> Ok.  The difference between irqfd and the ioctl is that with irqfd
> everything (the payload in your case, the interrupt number for others)
> is prepared beforehand, while with the ioctl the extra information is
> delivered with the ioctl.

This should work for us.

Cornelia