Re: [PATCH v4 4/4] pci_assign: Flip defaults of prefer_msi and share_intx

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 08, 2012 at 11:10:27AM +0100, Jan Kiszka wrote:
> INTx sharing is a bit more expensive than exclusive host interrupts, but
> this channel is not supposed to be used for high-performance scenarios
> anyway. Modern devices support MSI/MSI-X and do not depend on using INTx
> under critical workload, real old devices do not support INTx sharing
> anyway.
> 
> For those in the middle, the user experience is much better if they just
> work even when IRQ sharing is required. If there is nothing to share,
> share_intx=off can still be applied as tuning parameter.
> 
> With INTx sharing as default, the primary reason for prefer_msi=on is
> gone. Make it default off, specifically as it is known to cause troubles
> with devices that have incomplete/broken MSI support or otherwise
> stumble if host IRQ configuration does not match guest driver
> expectation.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

I'm not familiar enough with what prefer_msi does
so no comment here. Maybe Alex can ack.
If not I'll try to find the time to understand it
a bit better but will take some time.

>  hw/device-assignment.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/device-assignment.c b/hw/device-assignment.c
> index b7cabd4..caa3602 100644
> --- a/hw/device-assignment.c
> +++ b/hw/device-assignment.c
> @@ -1770,9 +1770,9 @@ static Property da_properties[] =
>      DEFINE_PROP_BIT("iommu", AssignedDevice, features,
>                     ASSIGNED_DEVICE_USE_IOMMU_BIT, true),
>      DEFINE_PROP_BIT("prefer_msi", AssignedDevice, features,
> -                   ASSIGNED_DEVICE_PREFER_MSI_BIT, true),
> +                    ASSIGNED_DEVICE_PREFER_MSI_BIT, false),
>      DEFINE_PROP_BIT("share_intx", AssignedDevice, features,
> -                    ASSIGNED_DEVICE_SHARE_INTX_BIT, false),
> +                    ASSIGNED_DEVICE_SHARE_INTX_BIT, true),
>      DEFINE_PROP_INT32("bootindex", AssignedDevice, bootindex, -1),
>      DEFINE_PROP_STRING("configfd", AssignedDevice, configfd_name),
>      DEFINE_PROP_END_OF_LIST(),
> -- 
> 1.7.3.4