在 2021/3/18 下午8:04, Eugenio Perez Martin 写道:
On Thu, Mar 18, 2021 at 11:48 AM Eugenio Perez Martin
<eperezma@xxxxxxxxxx> wrote:
On Thu, Mar 18, 2021 at 10:29 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
在 2021/3/18 下午5:18, Eugenio Perez Martin 写道:
On Thu, Mar 18, 2021 at 4:11 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
在 2021/3/18 上午12:47, Eugenio Perez Martin 写道:
On Wed, Mar 17, 2021 at 3:05 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
在 2021/3/16 下午6:31, Eugenio Perez Martin 写道:
On Tue, Mar 16, 2021 at 8:18 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
在 2021/3/16 上午3:48, Eugenio Pérez 写道:
Shadow virtqueue notifications forwarding is disabled when vhost_dev
stops, so code flow follows usual cleanup.
Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
---
hw/virtio/vhost-shadow-virtqueue.h | 7 ++
include/hw/virtio/vhost.h | 4 +
hw/virtio/vhost-shadow-virtqueue.c | 113 ++++++++++++++++++++++-
hw/virtio/vhost.c | 143 ++++++++++++++++++++++++++++-
4 files changed, 265 insertions(+), 2 deletions(-)
diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
index 6cc18d6acb..c891c6510d 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h
@@ -17,6 +17,13 @@
typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;
+bool vhost_shadow_vq_start(struct vhost_dev *dev,
+ unsigned idx,
+ VhostShadowVirtqueue *svq);
+void vhost_shadow_vq_stop(struct vhost_dev *dev,
+ unsigned idx,
+ VhostShadowVirtqueue *svq);
+
VhostShadowVirtqueue *vhost_shadow_vq_new(struct vhost_dev *dev, int idx);
void vhost_shadow_vq_free(VhostShadowVirtqueue *vq);
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index ac963bf23d..7ffdf9aea0 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -55,6 +55,8 @@ struct vhost_iommu {
QLIST_ENTRY(vhost_iommu) iommu_next;
};
+typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;
+
typedef struct VhostDevConfigOps {
/* Vhost device config space changed callback
*/
@@ -83,7 +85,9 @@ struct vhost_dev {
uint64_t backend_cap;
bool started;
bool log_enabled;
+ bool shadow_vqs_enabled;
uint64_t log_size;
+ VhostShadowVirtqueue **shadow_vqs;
Any reason that you don't embed the shadow virtqueue into
vhost_virtqueue structure?
Not really, it could be relatively big and I would prefer SVQ
members/methods to remain hidden from any other part that includes
vhost.h. But it could be changed, for sure.
(Note that there's a masked_notifier in struct vhost_virtqueue).
They are used differently: in SVQ the masked notifier is a pointer,
and if it's NULL the SVQ code knows that device is not masked. The
vhost_virtqueue is the real owner.
Yes, but it's an example for embedding auxciliary data structures in the
vhost_virtqueue.
It could be replaced by a boolean in SVQ or something like that, I
experimented with a tri-state too (UNMASKED, MASKED, MASKED_NOTIFIED)
and let vhost.c code to manage all the transitions. But I find clearer
the pointer use, since it's the more natural for the
vhost_virtqueue_mask, vhost_virtqueue_pending existing functions.
This masking/unmasking is the part I dislike the most from this
series, so I'm very open to alternatives.
See below. I think we don't even need to care about that.
Error *migration_blocker;
const VhostOps *vhost_ops;
void *opaque;
diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 4512e5b058..3e43399e9c 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -8,9 +8,12 @@
*/
#include "hw/virtio/vhost-shadow-virtqueue.h"
+#include "hw/virtio/vhost.h"
+
+#include "standard-headers/linux/vhost_types.h"
#include "qemu/error-report.h"
-#include "qemu/event_notifier.h"
+#include "qemu/main-loop.h"
/* Shadow virtqueue to relay notifications */
typedef struct VhostShadowVirtqueue {
@@ -18,14 +21,121 @@ typedef struct VhostShadowVirtqueue {
EventNotifier kick_notifier;
/* Shadow call notifier, sent to vhost */
EventNotifier call_notifier;
+
+ /*
+ * Borrowed virtqueue's guest to host notifier.
+ * To borrow it in this event notifier allows to register on the event
+ * loop and access the associated shadow virtqueue easily. If we use the
+ * VirtQueue, we don't have an easy way to retrieve it.
So this is something that worries me. It looks like a layer violation
that makes the codes harder to work correctly.
I don't follow you here.
The vhost code already depends on virtqueue in the same sense:
virtio_queue_get_host_notifier is called on vhost_virtqueue_start. So
if this behavior ever changes it is unlikely for vhost to keep working
without changes. vhost_virtqueue has a kick/call int where I think it
should be stored actually, but they are never used as far as I see.
Previous RFC did rely on vhost_dev_disable_notifiers. From its documentation:
/* Stop processing guest IO notifications in vhost.
* Start processing them in qemu.
...
But it was easier for this mode to miss a notification, since they
create a new host_notifier in virtio_bus_set_host_notifier right away.
So I decided to use the file descriptor already sent to vhost in
regular operation mode, so guest-related resources change less.
Having said that, maybe it's useful to assert that
vhost_dev_{enable,disable}_notifiers are never called on shadow
virtqueue mode. Also, it could be useful to retrieve it from
virtio_bus, not raw shadow virtqueue, so all get/set are performed
from it. Would that make more sense?
I wonder if it would be simpler to start from a vDPA dedicated shadow
virtqueue implementation:
1) have the above fields embeded in vhost_vdpa structure
2) Work at the level of
vhost_vdpa_set_vring_kick()/vhost_vdpa_set_vring_call()
This notifier is never sent to the device in shadow virtqueue mode.
It's for SVQ to react to guest's notifications, registering it on its
main event loop [1]. So if I perform these changes the way I
understand them, SVQ would still rely on this borrowed EventNotifier,
and it would send to the vDPA device the newly created kick_notifier
of VhostShadowVirtqueue.
The point is that vhost code should be coupled loosely with virtio. If
you try to "borrow" EventNotifier from virtio, you need to deal with a
lot of synchrization. An exampleis the masking stuffs.
I still don't follow this, sorry.
The svq->host_notifier event notifier is not affected by the masking
issue, it is completely private to SVQ. This commit creates and uses
it, and nothing related to masking is touched until the next commit.
Then the layer is still isolated and you have a much simpler context to
work that you don't need to care a lot of synchornization:
1) vq masking
This EventNotifier is not used for masking, it does not change from
the start of the shadow virtqueue operation through its end. Call fd
sent to vhost/vdpa device does not change either in shadow virtqueue
mode operation with masking/unmasking. I will try to document it
better.
I think that we will need to handle synchronization with
masking/unmasking from the guest and dynamically enabling SVQ
operation mode, since they can happen at the same time as long as we
let the guest run. There may be better ways of synchronizing them of
course, but I don't see how moving to the vhost-vdpa backend helps
with this. Please expand if I've missed it.
Or do you mean to forbid regular <-> SVQ operation mode transitions and delay it
to future patchsets?
So my idea is to do all the shadow virtqueue in the vhost-vDPA codes and
hide them from the upper layers like virtio. This means it works at
vhost level which can see vhost_vring_file only. When enalbed, what it
needs is just:
1) switch to use svq kickfd and relay ioeventfd to svq kickfd
2) switch to use svq callfd and relay svq callfd to irqfd
It will still behave like a vhost-backend that the switching is done
internally in vhost-vDPA which is totally transparent to the virtio
codes of Qemu.
E.g:
1) in the case of guest notifier masking, we don't need to do anything
since virtio codes will replace another irqfd for us.
Assuming that we don't modify vhost masking code, but send shadow
virtqueue call descriptor to the vhost device:
If guest virtio code mask the virtqueue and replaces the vhost-vdpa
device call fd (VhostShadowVirtqueue.call_notifier in the next commit,
or the descriptor in your previous second point, svq callfd) with the
masked notifier, vhost_shadow_vq_handle_call will not be called
anymore, and no more used descriptors will be forwarded. They will be
stuck if the shadow virtqueue forever. Guest itself cannot recover
from this situation, since a masking will set irqfd, not SVQ call fd.
Just to make sure we're in the same page. During vq masking, the virtio
codes actually use the masked_notifier as callfd in vhost_virtqueue_mask():
if (mask) {
assert(vdev->use_guest_notifier_mask);
file.fd = event_notifier_get_fd(&hdev->vqs[index].masked_notifier);
} else {
file.fd = event_notifier_get_fd(virtio_queue_get_guest_notifier(vvq));
}
file.index = hdev->vhost_ops->vhost_get_vq_index(hdev, n);
r = hdev->vhost_ops->vhost_set_vring_call(hdev, &file);
So consider the shadow virtqueue in done at vhost-vDPA. We just need to
make sure
1) update the callfd which passed by virtio layer via set_vring_kick()
2) always write to the callfd during vhost_shadow_vq_handle_call()
Then
3) When shadow vq is enabled, we just set the callfd of shadow virtqueue
to vDPA via VHOST_SET_VRING_CALL, and poll the svq callfd
4) When shadow vq is disabled, we just set the callfd that is passed by
virtio via VHOST_SET_VRING_CALL, stop poll the svq callfd
So you can see in step 2 and 4, we don't need to know whether or not the
vq is masked since we follow the vhost protocol "VhostOps" and do
everyhing transparently in the vhost-(vDPA) layer.
All of this assumes that we can enable/disable SVQ dynamically while
the device is running. If it's not the case, there is no need for the
mutex neither in vhost.c code nor vdpa_backend.
As I see it, the issue is that step (2) and (4) happens in different
threads: (2) is in vCPU vmexit, and (4) is in main event loop.
Consider unmasking and disabling SVQ at the same time with no mutex:
vCPU vmexit thread aio thread
(unmask) (stops SVQ)
| |
| // Last callfd set was masked_notifier
| vdpa_backend.callfd = \
| atomic_read(masked_notifier).
| |
vhost_set_vring_call(vq.guest_notifier)|
-> vdpa_backend.callfd = \ |
vq.guest_notifier |
| |
| ioctl(vdpa,
VHOST_SET_VRING_CALL, vdpa_backend.callfd)
|
// guest expects more interrupts, but
// device just set masked
And vhost_set_vring_call could happen entirely even while ioctl is
being executed.
So that is the reason for the mutex: vdpa_backend.call_fd and the
ioctl VHOST_SET_VRING_CALL must be serialized. I'm ok with moving to
vdpa backend, but it's the same code, just in vdpa_backend.c instead
of vhost.c, so it becomes less generic in my opinion.
You are right. But let's consider if we can avoid the dedicated mutex.
E.g can we use the BQL, bascially we need to synchronizae with iothread.
Or is it possible to schedule bh then things are serailzied automatically?
I tried RCU with no success, and I think the same issues apply to bh.
I will try to explain the best I can what I achieved in the past, and
why I discarded it. I will explore BQL approaches, it could be simpler
that way actually.
The hard part to achieve if that no notification can be forwarded to
the guest once masking vmexit returns (isn't it?). Unmasking scenario
is easy with RCU, since the pending notification could reach the guest
asynchronously if it exists.
On the other hand, whatever guest set should take priority over
whatever shadow_vq_stop sets.
With RCU, the problem is that the synchronization point should be the
vmexit thread. The function vhost_virtqueue_mask is already called
within RCU, so it could happen that RCU lock is held longer than it
returns, so the effective masking could happen after vmexit returns. I
see no way to make something like "call_rcu but only in this thread"
or, in other words, " rcu_synchronize after the rcu_unlock of this
thread and then run this".
I tried to explore to synchronize that situation in the event loop,
but the guest is able to call unmask/mask again, making the race
condition. If I can mark a range where unmask/mask cannot return, I'm
creating an artificial mutex. If I retry in the main event loop, there
is a window where notification can reach the guest after masking.
In order to reduce this window, shadow_virtqueue_stop could set the
masked notifier fd unconditionally, and then check if it should unmask
under the mutex. I'm not sure if this is worth however, since
enabling/disabling already involves a system call.
I think it would be way more natural to at least protect
vhost_virtqueue.notifier_is_masked with the BQL, however, so I will
check that possibility. It would be great to be able to do it on bh,
but I think this opens a window for the host to send notifications
when the guest has masked the vq, since mask/unmasking could happen
while bh is running as far as I see.
So actually vhost_shadow_virtqueue_start/stop (being a QMP command)
and vhost_virtqueue_mask (vmexit) already runs under BQL.
So actually everyhing is alreay synchornized?
1) MSI-X MMIO handlers from vcpu thread with BQL
2) Poll for ioeventfd and vhost callfd from iothread with BQL
3) the monitor comand with BQL held
The ideal scenario would be to run kick/call handlers in its own aio
context and do not take BQL on them,
Well, then you still need to synchronize with QMP, MSI-X MMIO.
but I think this is doable with
just atomics and maybe events. For this series I think we can delete
the introduced mutex (and maybe replace it with an assertion?)
Let's try to avoid assertion here.
Thanks
Thanks!
Actually, in my test I override vhost_virtqueue_mask, so it looks more
similar to your proposal with VhostOps.
Thanks!
2) easily to deal with vhost dev start and stop
The advantages are obvious, simple and easy to implement.
I still don't see how performing this step from backend code avoids
the synchronization problem, since they will be done from different
threads anyway. Not sure what piece I'm missing.
See my reply in another thread. If you enable the shadow virtqueue via a
OOB monitor that's a real issue.
But I don't think we need to do that since
1) SVQ should be transparnet to management
2) unncessary synchronization issue
We can enable the shadow virtqueue through cli, new parameter with
vhost-vdpa probably. Then we don't need to care about threads. And in
the final version with full live migration support, the shadow virtqueue
should be enabled automatically. E.g for the device without
VHOST_F_LOG_ALL or we can have a dedicated capability of vDPA via
VHOST_GET_BACKEND_FEATURES.
It should be enabled automatically in those condition, but it also
needs to be dynamic, and only be active during migration. Otherwise,
guest should use regular vdpa operation. The problem with masking is
the same if we enable with QMP or because live migration event.
So we will have the previous synchronization problem sooner or later.
If we omit the rollback to regular vdpa operation (in other words,
disabling SVQ), code can be simplified, but I'm not sure if that is
desirable.
Rgiht, so I'm ok to have the synchronziation from the start if you wish.
But we need to figure out what to synchronize and how to do synchronize.
THanks
Thanks!
Thanks
I can see / tested a few solutions but I don't like them a lot:
* Forbid hot-swapping from/to shadow virtqueue mode, and set it from
cmdline: We will have to deal with setting the SVQ mode dynamically
sooner or later if we want to use it for live migration.
* Forbid coming back to regular mode after switching to shadow
virtqueue mode: The heavy part of the synchronization comes from svq
stopping code, since we need to serialize the setting of device call
fd. This could be acceptable, but I'm not sure about the implications:
What happens if live migration fails and we need to step back? A mutex
is not needed in this scenario, it's ok with atomics and RCU code.
* Replace KVM_IRQFD instead and let SVQ poll the old one and masked
notifier: I haven't thought a lot of this one, I think it's better to
not touch guest notifiers.
* Monitor also masked notifier from SVQ: I think this could be
promising, but SVQ needs to be notified about masking/unmasking
anyway, and there is code that depends on checking the masked notifier
for the pending notification.
2) vhost dev start and stop
?
+ *
+ * So shadow virtqueue must not clean it, or we would lose VirtQueue one.
+ */
+ EventNotifier host_notifier;
+
+ /* Virtio queue shadowing */
+ VirtQueue *vq;
} VhostShadowVirtqueue;
+/* Forward guest notifications */
+static void vhost_handle_guest_kick(EventNotifier *n)
+{
+ VhostShadowVirtqueue *svq = container_of(n, VhostShadowVirtqueue,
+ host_notifier);
+
+ if (unlikely(!event_notifier_test_and_clear(n))) {
+ return;
+ }
+
+ event_notifier_set(&svq->kick_notifier);
+}
+
+/*
+ * Restore the vhost guest to host notifier, i.e., disables svq effect.
+ */
+static int vhost_shadow_vq_restore_vdev_host_notifier(struct vhost_dev *dev,
+ unsigned vhost_index,
+ VhostShadowVirtqueue *svq)
+{
+ EventNotifier *vq_host_notifier = virtio_queue_get_host_notifier(svq->vq);
+ struct vhost_vring_file file = {
+ .index = vhost_index,
+ .fd = event_notifier_get_fd(vq_host_notifier),
+ };
+ int r;
+
+ /* Restore vhost kick */
+ r = dev->vhost_ops->vhost_set_vring_kick(dev, &file);
+ return r ? -errno : 0;
+}
+
+/*
+ * Start shadow virtqueue operation.
+ * @dev vhost device
+ * @hidx vhost virtqueue index
+ * @svq Shadow Virtqueue
+ */
+bool vhost_shadow_vq_start(struct vhost_dev *dev,
+ unsigned idx,
+ VhostShadowVirtqueue *svq)
It looks to me this assumes the vhost_dev is started before
vhost_shadow_vq_start()?
Right.
This might not true. Guest may enable and disable virtio drivers after
the shadow virtqueue is started. You need to deal with that.
Right, I will test this scenario.
Thanks
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