Re: [RFC QEMU PATCH v3 1/1] xen: Use gsi instead of irq for mapping pirq

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Dec 11, 2023 at 12:52:40AM +0800, Jiqian Chen wrote:
> In PVH dom0, it uses the linux local interrupt mechanism,
> when it allocs irq for a gsi, it is dynamic, and follow
> the principle of applying first, distributing first. And
> the irq number is alloced from small to large, but the
> applying gsi number is not, may gsi 38 comes before gsi
> 28, that causes the irq number is not equal with the gsi
> number. And when passthrough a device, qemu wants to use
> gsi to map pirq, xen_pt_realize->xc_physdev_map_pirq, but
> the gsi number is got from file
> /sys/bus/pci/devices/<sbdf>/irq in current code, so it
> will fail when mapping.
> 
> Use real gsi number read from gsi sysfs.
> 
> Co-developed-by: Huang Rui <ray.huang@xxxxxxx>
> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
> ---
>  hw/xen/xen-host-pci-device.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/xen/xen-host-pci-device.c b/hw/xen/xen-host-pci-device.c
> index 8c6e9a1716..e270ac2631 100644
> --- a/hw/xen/xen-host-pci-device.c
> +++ b/hw/xen/xen-host-pci-device.c
> @@ -364,7 +364,7 @@ void xen_host_pci_device_get(XenHostPCIDevice *d, 
> uint16_t domain,
>      }
>      d->device_id = v;
>  
> -    xen_host_pci_get_dec_value(d, "irq", &v, errp);
> +    xen_host_pci_get_dec_value(d, "gsi", &v, errp);

Don't you need to fallthrough to use the irq number on failure?
Otherwise passthrough won't work on older Linux versions that don't
expose the gsi node.

Thanks, Roger.