vhost: Don't set vring call if guest notifier is unused

The vring call fd is set even when the guest does not use MSI-X (e.g., in the
case of virtio PMD), leading to unnecessary CPU overhead for processing
interrupts.

The commit 96a3d98d2c("vhost: don't set vring call if no vector") optimized the
case where MSI-X is enabled but the queue vector is unset. However, there's an
additional case where the guest uses INTx and the INTx_DISABLED bit in the PCI
config is set, meaning that no interrupt notifier will actually be used.

In such cases, the vring call fd should also be cleared to avoid redundant
interrupt handling.

Fixes: 96a3d98d2c("vhost: don't set vring call if no vector")
Reported-by: Zhiyuan Yuan <yuanzhiyuan@chinatelecom.cn>
Signed-off-by: Jidong Xia <xiajd@chinatelecom.cn>
Signed-off-by: Huaitong Han <hanht2@chinatelecom.cn>
Message-Id: <20250522100548.212740-1-hanht2@chinatelecom.cn>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index c6b5768f3a5ddccb66529be0176c7e7d5d21b5a0..9b4bf48439ed225e6b443f067928a84950f12a88 100644 (file)
--- a/hw/pci/pci.c
+++ b/hw/pci/pci.c
@@ -1730,7 +1730,7 @@ static void pci_update_mappings(PCIDevice *d)
     pci_update_vga(d);
 }
 
-static inline int pci_irq_disabled(PCIDevice *d)
+int pci_irq_disabled(PCIDevice *d)
 {
     return pci_get_word(d->config + PCI_COMMAND) & PCI_COMMAND_INTX_DISABLE;
 }

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 9b48aa8c3e7b590f9fadcbfde2a35d80f9712a83..7e309d1d49ef557d27a276b1cb7157c9fd5c917e 100644 (file)
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1215,7 +1215,12 @@ static int virtio_pci_set_guest_notifier(DeviceState *d, int n, bool assign,
 static bool virtio_pci_query_guest_notifiers(DeviceState *d)
 {
     VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
-    return msix_enabled(&proxy->pci_dev);
+
+    if (msix_enabled(&proxy->pci_dev)) {
+        return true;
+    } else {
+        return pci_irq_disabled(&proxy->pci_dev);
+    }
 }
 
 static int virtio_pci_set_guest_notifiers(DeviceState *d, int nvqs, bool assign)

diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
index a6854dad2b08c3e4157319da35f7157f7077e1b1..35d59d7672e2b9ca43ae2f9708e94a07c04f9784 100644 (file)
--- a/include/hw/pci/pci.h
+++ b/include/hw/pci/pci.h
@@ -983,6 +983,7 @@ void lsi53c8xx_handle_legacy_cmdline(DeviceState *lsi_dev);
 
 qemu_irq pci_allocate_irq(PCIDevice *pci_dev);
 void pci_set_irq(PCIDevice *pci_dev, int level);
+int pci_irq_disabled(PCIDevice *d);
 
 static inline void pci_irq_assert(PCIDevice *pci_dev)
 {