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Date: Tue, 25 Jan 2011 16:13:01 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Message-ID: <20110125141301.GA15666@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] virtio-pci: Disable virtio-ioeventfd when
 !CONFIG_IOTHREAD
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org

On Tue, Jan 25, 2011 at 01:58:08PM +0000, Stefan Hajnoczi wrote:
> It is not possible to use virtio-ioeventfd when building without an I/O
> thread.  We rely on a signal to kick us out of vcpu execution.  Timers
> and AIO use SIGALRM and SIGUSR2 respectively.  Unfortunately eventfd
> does not support O_ASYNC (SIGIO) so eventfd cannot be used in a signal
> driven manner.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

I'd rather have the CONFIG_ ... mess contained in kvm-all.c
which tests CONFIG_IOTHREAD anyway, than spread out
to devices. Can we make kvm_has_many_ioeventfds check this?

> ---
>  hw/virtio-pci.c |   21 ++++++++++++++++++++-
>  1 files changed, 20 insertions(+), 1 deletions(-)
> 
> diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
> index d07ff97..e921eda 100644
> --- a/hw/virtio-pci.c
> +++ b/hw/virtio-pci.c
> @@ -640,6 +640,25 @@ static int virtio_pci_set_host_notifier(void *opaque, int n, bool assign)
>      return virtio_pci_set_host_notifier_internal(proxy, n, assign);
>  }
>  
> +static bool virtio_pci_can_use_ioeventfd(void)
> +{
> +    if (!kvm_has_many_ioeventfds()) {
> +        return false;
> +    }
> +
> +    /* Use ioeventfd for virtqueue kick only if we have an I/O thread to
> +     * perform out-of-line processing.  Otherwise we might as well do
> +     * synchronous virtqueue kicks and in fact we have to since eventfd does
> +     * not support SIGIO.  Without the I/O thread a signal would be required to
> +     * kick the vcpu out of guest code.
> +     */
> +#ifdef CONFIG_IOTHREAD
> +    return true;
> +#else
> +    return false;
> +#endif
> +}
> +
>  static void virtio_pci_vmstate_change(void *opaque, bool running)
>  {
>      VirtIOPCIProxy *proxy = opaque;
> @@ -705,7 +724,7 @@ static void virtio_init_pci(VirtIOPCIProxy *proxy, VirtIODevice *vdev,
>      pci_register_bar(&proxy->pci_dev, 0, size, PCI_BASE_ADDRESS_SPACE_IO,
>                             virtio_map);
>  
> -    if (!kvm_has_many_ioeventfds()) {
> +    if (!virtio_pci_can_use_ioeventfd()) {
>          proxy->flags &= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
>      }
>  
> -- 
> 1.7.2.3