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Re: [Qemu-devel] [PATCH V2] virtio: Fix no interrupt when not creating m

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH V2] virtio: Fix no interrupt when not creating msi controller
Date: Tue, 24 Jan 2017 16:44:02 +0200

On Tue, Jan 24, 2017 at 10:25:00AM +0100, Paolo Bonzini wrote:
> On 23/01/2017 18:04, Michael S. Tsirkin wrote:
> >> For virtio-blk, my patch moved the setting of ISR from
> >> virtio_queue_guest_notifier_read to virtio_notify_irqfd.  This is
> >> because the irqfd emulation only needs to trigger the interrupt.
> >> Setting the ISR should have been done elsewhere.
> >>
> >> Can vhost set ISR (I thought it couldn't)?  If so, it seems like ARM was
> >> relying on QEMU's irqfd emulation, as a work around for vhost not
> >> setting ISR.  But this only works if irqfd is directed to QEMU and not
> >> to KVM.  So if vhost cannot set ISR, I think vhost should be disabled
> >> unless MSI is active.
> >>
> >> Am I missing something?
> >
> > vhost doesn't set ISR ATM.  Without MSI we are always bouncing
> > interrupts through QEMU in particular in order to set ISR.
> Where is the code that disables KVM irqfd?
> Paolo

This bit in virtio_pci_set_guest_notifiers does the trick I think:

    bool with_irqfd = msix_enabled(&proxy->pci_dev) &&

I don't think we ever used irqfd for level interrupts.

> > Disabling vhost when user requested it was what we did
> > at some point but this just confused people.
> > 
> > 

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