[v17,01/11] hw/usb/hcd-xhci-pci: Use modulo to select MSI vector as per spec

Commit Message

Phil Dennis-Jordan Jan. 12, 2025, 9 p.m. UTC

QEMU would crash with a failed assertion if the XHCI controller
attempted to raise the interrupt on an interrupter corresponding
to a MSI vector with a higher index than the highest configured
for the device by the guest driver.

This behaviour is correct on the MSI/PCI side: per PCI 3.0 spec,
devices must ensure they do not send MSI notifications for
vectors beyond the range of those allocated by the system/driver
software. Unlike MSI-X, there is no generic way for handling
aliasing in the case of fewer allocated vectors than requested,
so the specifics are up to device implementors. (Section
6.8.3.4. "Sending Messages")

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. This makes the device work correctly in MSI mode
with macOS's XHCI driver, which only allocates a single vector.

Signed-off-by: Phil Dennis-Jordan <[e-mail]>
Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
---

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(+)

Patch

diff --git a/hw/usb/hcd-xhci-pci.c b/hw/usb/hcd-xhci-pci.c
index a069b42338..49642aab58 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);
         return true;
     }