[PATCH V2] vhost: Don't set vring call if guest notifier is unused

[oenhan]

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>
---
V2:
- Retain the name `query_guest_notifiers`
- All qtest/unit test cases pass
- Fix V1 patch style problems

 hw/pci/pci.c                   |  2 +-
 hw/s390x/virtio-ccw.c          |  7 +++++--
 hw/virtio/vhost.c              |  3 +--
 hw/virtio/virtio-pci.c         | 10 ++++++++--
 include/hw/pci/pci.h           |  1 +
 include/hw/virtio/virtio-bus.h |  2 +-
 6 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index 352b3d12c8..45b491412a 100644
--- a/hw/pci/pci.c
+++ b/hw/pci/pci.c
@@ -1712,7 +1712,7 @@ 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/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index d2f85b39f3..632708ba4d 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -936,11 +936,14 @@ static void virtio_ccw_vmstate_change(DeviceState *d, 
bool running)
     }
 }
 
-static bool virtio_ccw_query_guest_notifiers(DeviceState *d)
+static bool virtio_ccw_query_guest_notifiers(DeviceState *d, int n)
 {
     CcwDevice *dev = CCW_DEVICE(d);
+    VirtioCcwDevice *vdev = VIRTIO_CCW_DEVICE(d);
+    VirtIODevice *virtio_dev = virtio_bus_get_device(&vdev->bus);
 
-    return !!(dev->sch->curr_status.pmcw.flags & PMCW_FLAGS_MASK_ENA);
+    return !!(dev->sch->curr_status.pmcw.flags & PMCW_FLAGS_MASK_ENA)
+            && virtio_queue_vector(virtio_dev, n) != VIRTIO_NO_VECTOR;
 }
 
 static int virtio_ccw_get_mappings(VirtioCcwDevice *dev)
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 4cae7c1664..2a9a839763 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1341,8 +1341,7 @@ int vhost_virtqueue_start(struct vhost_dev *dev,
     }
 
     if (k->query_guest_notifiers &&
-        k->query_guest_notifiers(qbus->parent) &&
-        virtio_queue_vector(vdev, idx) == VIRTIO_NO_VECTOR) {
+        !k->query_guest_notifiers(qbus->parent, idx)) {
         file.fd = -1;
         r = dev->vhost_ops->vhost_set_vring_call(dev, &file);
         if (r) {
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 0fa8fe4955..d62e199489 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1212,10 +1212,16 @@ static int virtio_pci_set_guest_notifier(DeviceState 
*d, int n, bool assign,
     return 0;
 }
 
-static bool virtio_pci_query_guest_notifiers(DeviceState *d)
+static bool virtio_pci_query_guest_notifiers(DeviceState *d, int n)
 {
     VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
-    return msix_enabled(&proxy->pci_dev);
+    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+
+    if (msix_enabled(&proxy->pci_dev)) {
+        return virtio_queue_vector(vdev, n) != VIRTIO_NO_VECTOR;
+    } 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 c2fe6caa2c..8c24bd97db 100644
--- a/include/hw/pci/pci.h
+++ b/include/hw/pci/pci.h
@@ -668,6 +668,7 @@ 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/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
index 7ab8c9dab0..75d43b508a 100644
--- a/include/hw/virtio/virtio-bus.h
+++ b/include/hw/virtio/virtio-bus.h
@@ -48,7 +48,7 @@ struct VirtioBusClass {
     int (*load_done)(DeviceState *d, QEMUFile *f);
     int (*load_extra_state)(DeviceState *d, QEMUFile *f);
     bool (*has_extra_state)(DeviceState *d);
-    bool (*query_guest_notifiers)(DeviceState *d);
+    bool (*query_guest_notifiers)(DeviceState *d, int n);
     int (*set_guest_notifiers)(DeviceState *d, int nvqs, bool assign);
     int (*set_host_notifier_mr)(DeviceState *d, int n,
                                 MemoryRegion *mr, bool assign);
-- 
2.43.5