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Re: [Qemu-devel] [PATCH] vhost: force vhost off for non-MSI guests

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] vhost: force vhost off for non-MSI guests
Date: Mon, 14 Mar 2011 21:00:02 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, Mar 14, 2011 at 10:35:08PM +0530, rukhsana ansari wrote:
> Seeking clarification to the original question I posted:
> >>
> >>
> > This maybe a novice question - Would appreciate it if you can you provide a
> > pointer to documentation or relevant code that explains what is the
> > limitation in supporting level irq support in kvm irqfd.
> >
> >
> >
> After browsing the KVM kernel code, it does look like direct assignment of PCI
> devices allows support for level-triggered interrupts to be injected to the
> guest from the kernel.  (as opposed to not supporting it for vhost irqfd
> mechanism)
> This occurs when the guest device supports INTX.
> Reference:  kvm_assigned_dev_interrupt_work_handler() in assigned-dev.c calls
> kvm_set_irq()
> with the guest_irq.
> This function in turn invokes the assigned set function  (either
> kvm_set_pic_irq or kvm_set_ioapic_irq) which was setup at kvm_irq_chip 
> creation
> time when kvm_setup_default_irq_routing () called for handling ioctl
> So, it isn't clear why level-triggered interrupt isn't supported for irqfd
> mechanism.
> Would greatly appreciate clarification here
> Thanks
> -Rukhsana

Mostly, no one came up with an implementation so far.

If the point is to use irqfd with vhost-net, there's also
a question of adding interfaces to
1. pass IO read transactions directly to another kernel module
2. add an interface to clear the irq level

Maybe the right thing is to combine the two somehow:
irqfd might get an oiption to set a bit in memory,
ioeventfd might get an option to read and clear from memory
and clear irqfd line at the same time.


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