If you happen to have a kernel with ioeventfd support enabled, but
missing support for them in KVM, and you attempt to enable vhost by
setting vhost=on, qemu aborts with error:
kvm_mem_ioeventfd_add: error adding ioeventfd: Function not implemented
This patch adds a mechanism which allows to emulate KVM binding by
triggering the related notifiers via the userspace. The first time the
emulation is used, a warning is displayed, so that the user knows about
potential performance impact:
2015-11-19T09:35:16.618380Z qemu-system-aarch64: KVM does not support eventfd
binding, using userspace event forwarding (slow)
This problem can be observed with libvirt, which checks for /dev/vhost-net
availability and just inserts "vhost=on" automatically in this case; on an
ARM64 system using stock kernel 3.18.0 with CONFIG_IOEVENTFD enabled in
expert settings.
Signed-off-by: Pavel Fedin <p.fe...@samsung.com>
---
hw/virtio/virtio-mmio.c | 15 +++++++++---
hw/virtio/virtio-pci.c | 61 ++++++++++++++++++++++++++--------------------
hw/virtio/virtio.c | 24 +++++++++++++++++-
include/hw/virtio/virtio.h | 1 +
4 files changed, 69 insertions(+), 32 deletions(-)
diff --git a/hw/virtio/virtio-mmio.c b/hw/virtio/virtio-mmio.c
index 16621fa..69d4cbc 100644
--- a/hw/virtio/virtio-mmio.c
+++ b/hw/virtio/virtio-mmio.c
@@ -110,11 +110,18 @@ static int
virtio_mmio_set_host_notifier_internal(VirtIOMMIOProxy *proxy,
return r;
}
virtio_queue_set_host_notifier_fd_handler(vq, true, set_handler);
- memory_region_add_eventfd(&proxy->iomem, VIRTIO_MMIO_QUEUENOTIFY, 4,
- true, n, notifier);
+
+ if (kvm_eventfds_enabled()) {
+ memory_region_add_eventfd(&proxy->iomem, VIRTIO_MMIO_QUEUENOTIFY,
4,
+ true, n, notifier);
+ } else if (!set_handler) {
+ virtio_queue_set_host_notifier_forwarding(vq);
+ }
} else {
- memory_region_del_eventfd(&proxy->iomem, VIRTIO_MMIO_QUEUENOTIFY, 4,
- true, n, notifier);
+ if (kvm_eventfds_enabled()) {
+ memory_region_del_eventfd(&proxy->iomem, VIRTIO_MMIO_QUEUENOTIFY,
4,
+ true, n, notifier);
+ }
virtio_queue_set_host_notifier_fd_handler(vq, false, false);
event_notifier_cleanup(notifier);
}
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 96be4fd..b27a630 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -293,41 +293,48 @@ static int
virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
return r;
}
virtio_queue_set_host_notifier_fd_handler(vq, true, set_handler);
- if (modern) {
- if (fast_mmio) {
- memory_region_add_eventfd(modern_mr, modern_addr, 0,
- false, n, notifier);
- } else {
- memory_region_add_eventfd(modern_mr, modern_addr, 2,
- false, n, notifier);
+
+ if (kvm_eventfds_enabled()) {
+ if (modern) {
+ if (fast_mmio) {
+ memory_region_add_eventfd(modern_mr, modern_addr, 0,
+ false, n, notifier);
+ } else {
+ memory_region_add_eventfd(modern_mr, modern_addr, 2,
+ false, n, notifier);
+ }
+ if (modern_pio) {
+ memory_region_add_eventfd(modern_notify_mr, 0, 2,
+ true, n, notifier);
+ }
}
- if (modern_pio) {
- memory_region_add_eventfd(modern_notify_mr, 0, 2,
- true, n, notifier);
+ if (legacy) {
+ memory_region_add_eventfd(legacy_mr, legacy_addr, 2,
+ true, n, notifier);
}
- }
- if (legacy) {
- memory_region_add_eventfd(legacy_mr, legacy_addr, 2,
- true, n, notifier);
+ } else if (!set_handler) {
+ virtio_queue_set_host_notifier_forwarding(vq);
}
} else {
- if (modern) {
- if (fast_mmio) {
- memory_region_del_eventfd(modern_mr, modern_addr, 0,
- false, n, notifier);
- } else {
- memory_region_del_eventfd(modern_mr, modern_addr, 2,
- false, n, notifier);
+ if (kvm_eventfds_enabled()) {
+ if (modern) {
+ if (fast_mmio) {
+ memory_region_del_eventfd(modern_mr, modern_addr, 0,
+ false, n, notifier);
+ } else {
+ memory_region_del_eventfd(modern_mr, modern_addr, 2,
+ false, n, notifier);
+ }
+ if (modern_pio) {
+ memory_region_del_eventfd(modern_notify_mr, 0, 2,
+ true, n, notifier);
+ }
}
- if (modern_pio) {
- memory_region_del_eventfd(modern_notify_mr, 0, 2,
+ if (legacy) {
+ memory_region_del_eventfd(legacy_mr, legacy_addr, 2,
true, n, notifier);
}
}
- if (legacy) {
- memory_region_del_eventfd(legacy_mr, legacy_addr, 2,
- true, n, notifier);
- }
virtio_queue_set_host_notifier_fd_handler(vq, false, false);
event_notifier_cleanup(notifier);
}
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 1edef59..6fe268f 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -89,6 +89,7 @@ struct VirtQueue
VirtIODevice *vdev;
EventNotifier guest_notifier;
EventNotifier host_notifier;
+ bool forward_host_notifier;
QLIST_ENTRY(VirtQueue) node;
};
@@ -969,7 +970,13 @@ void virtio_queue_notify_vq(VirtQueue *vq)
void virtio_queue_notify(VirtIODevice *vdev, int n)
{
- virtio_queue_notify_vq(&vdev->vq[n]);
+ VirtQueue *vq = &vdev->vq[n];
+
+ if (vq->forward_host_notifier) {
+ event_notifier_set(&vq->host_notifier);
+ } else {
+ virtio_queue_notify_vq(&vdev->vq[n]);
+ }
}
uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)
@@ -1715,6 +1722,21 @@ void virtio_queue_set_host_notifier_fd_handler(VirtQueue
*vq, bool assign,
/* Test and clear notifier before after disabling event,
* in case poll callback didn't have time to run. */
virtio_queue_host_notifier_read(&vq->host_notifier);
+
+ vq->forward_host_notifier = false;
+ }
+}
+
+static bool forwarding_warning;
+
+void virtio_queue_set_host_notifier_forwarding(VirtQueue *vq)
+{
+ vq->forward_host_notifier = true;
+
+ if (!forwarding_warning) {
+ forwarding_warning = true;
+ error_report("KVM does not support eventfd binding, "
+ "using userspace event forwarding (slow)");
}
}
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 205fadf..f288ccb 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -245,6 +245,7 @@ void virtio_queue_set_guest_notifier_fd_handler(VirtQueue
*vq, bool assign,
EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
void virtio_queue_set_host_notifier_fd_handler(VirtQueue *vq, bool assign,
bool set_handler);
+void virtio_queue_set_host_notifier_forwarding(VirtQueue *vq);
void virtio_queue_notify_vq(VirtQueue *vq);
void virtio_irq(VirtQueue *vq);
VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
--
1.9.5.msysgit.0
