Re: [PATCH v3 1/6] hw/usb/hcd-xhci-pci: Use modulo to select MSI vector as per spec

[Philippe Mathieu-Daudé]

On 27/12/24 13:13, Phil Dennis-Jordan wrote:
> QEMU would crash with a failed assertion if the XHCI controller
> attempted to raise the interrupt on a higher vector than the
> highest configured for the device by the guest driver.
>
> It turns out the XHCI spec (Implementation Note in section 4.17,
> "Interrupters") requires that the host controller signal the MSI
> vector with the number computed by taking the interrupter number
> modulo the number of enabled MSI vectors.
>
> This change introduces that modulo calculation, fixing the
> failed assertion and making the device work correctly in MSI mode
> with macOS's XHCI driver.
>
> Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
> ---
>
> v2:
>
>   * Switch to modulo arithmetic for MSI vector number rather than dropping,
>     as per spec.
>
>   hw/usb/hcd-xhci-pci.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/hw/usb/hcd-xhci-pci.c b/hw/usb/hcd-xhci-pci.c
> index e110840c7a..e5e7330387 100644
> --- a/hw/usb/hcd-xhci-pci.c
> +++ b/hw/usb/hcd-xhci-pci.c
> @@ -74,6 +74,7 @@ static bool xhci_pci_intr_raise(XHCIState *xhci, int n, bool 
> level)
>       }
>   
>       if (msi_enabled(pci_dev) && level) {
> +        n %= msi_nr_vectors_allocated(pci_dev);
>           msi_notify(pci_dev, n);

Should this be done at the MSI layer in the callee?
(I haven't checked the MSI spec).

(Cc'ing hw/pci/msi.c maintainers)

>           return true;
>       }