[PULL 23/31] vhost: Don't set vring call if guest notifier is unused

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

From: Huaitong Han <hanht2@chinatelecom.cn>

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>
---
 include/hw/pci/pci.h   | 1 +
 hw/pci/pci.c           | 2 +-
 hw/virtio/virtio-pci.c | 7 ++++++-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
index a6854dad2b..35d59d7672 100644
--- 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)
 {
diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index c6b5768f3a..9b4bf48439 100644
--- 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 9b48aa8c3e..7e309d1d49 100644
--- 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)
-- 
MST