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

[Huaitong Han]

Stefano Garzarella <sgarzare@redhat.com> 于2025年5月16日周五 16:19写道：
>
> On Tue, May 13, 2025 at 07:28:25PM +0800, oenhan@gmail.com wrote:
> >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)
>
> Since it was inline, will it be better to move the whole function to
> include/hw/pci/pci.h and keep it inline?
>
I did try moving the function to include/hw/pci/pci.h and marking it
inline, but ran into compilation issues due to the use of the incomplete
PCIDevice type.
Specifically, accessing d->config triggers the following error:
include/hw/pci/pci.h:674:26: error: invalid use of incomplete typedef
‘PCIDevice’
return pci_get_word(d->config + PCI_COMMAND) & PCI_COMMAND_INTX_DISABLE;
Including hw/pci/pci_device.h in pci.h to resolve this introduces
further issues, so I suggest to keep the function as a non-inline
helper in the .c file.

> Thanks,
> Stefano
>
> > {
> >     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
> >
>