Re: [PATCH] virtio: disable ioeventfd for record/replay

["Alex Bennée" <alex.bennee@linaro.org>]

Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru> writes:

> virtio devices support separate iothreads waiting for
> events from file descriptors. These are asynchronous
> events that can't be recorded and replayed, therefore
> this patch disables ioeventfd for all devices when
> record or replay is enabled.

Does it always have to be the case? Can a virtio device forgo iothreads
and poll events from the main loop?

>
> Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
> ---
>  hw/virtio/virtio-pci.c |    6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 094c36aa3e..76fbc111ea 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -37,6 +37,7 @@
>  #include "qemu/range.h"
>  #include "hw/virtio/virtio-bus.h"
>  #include "qapi/visitor.h"
> +#include "sysemu/replay.h"
>  
>  #define VIRTIO_PCI_REGION_SIZE(dev)     VIRTIO_PCI_CONFIG_OFF(msix_present(dev))
>  
> @@ -1746,6 +1747,11 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
>          proxy->flags &= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
>      }
>  
> +    /* fd-based ioevents can't be synchronized in record/replay */
> +    if (replay_mode != REPLAY_MODE_NONE) {
> +        proxy->flags &= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
> +    }
> +
>      /*
>       * virtio pci bar layout used by default.
>       * subclasses can re-arrange things if needed.

I think we need a better solution for this. Are we going to end up
sprinkling replay annotations throughout the device backends for each an
every device supported by QEMU?

This seems to be it should be the other way around. We should identify
the devices (or classes of devices) that support the logging and
consistency requirements needed for replay and mark them with a QOM
parameter. Then we can walk the tree of devices and either claim
compatibility with the request to record/replay or report to the user
"can't use record/replay because device %s is not supported".

-- 
Alex Bennée