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Re: [PATCH 1/4] vhost: Re-enable vrings after setting features

From: Anton Kuchin
Subject: Re: [PATCH 1/4] vhost: Re-enable vrings after setting features
Date: Thu, 13 Apr 2023 17:24:36 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 
On 13/04/2023 14:03, Stefan Hajnoczi wrote:

On Thu, 13 Apr 2023 at 04:20, Hanna Czenczek <hreitz@redhat.com> wrote:

On 12.04.23 22:51, Stefan Hajnoczi wrote:

On Tue, Apr 11, 2023 at 05:05:12PM +0200, Hanna Czenczek wrote:

If the back-end supports the VHOST_USER_F_PROTOCOL_FEATURES feature,
setting the vhost features will set this feature, too.  Doing so
disables all vrings, which may not be intended.

For example, enabling or disabling logging during migration requires
setting those features (to set or unset VHOST_F_LOG_ALL), which will
automatically disable all vrings.  In either case, the VM is running
(disabling logging is done after a failed or cancelled migration, and
only once the VM is running again, see comment in
memory_global_dirty_log_stop()), so the vrings should really be enabled.
As a result, the back-end seems to hang.

To fix this, we must remember whether the vrings are supposed to be
enabled, and, if so, re-enable them after a SET_FEATURES call that set
VHOST_USER_F_PROTOCOL_FEATURES.

It seems less than ideal that there is a short period in which the VM is
running but the vrings will be stopped (between SET_FEATURES and
SET_VRING_ENABLE).  To fix this, we would need to change the protocol,
e.g. by introducing a new flag or vhost-user protocol feature to disable
disabling vrings whenever VHOST_USER_F_PROTOCOL_FEATURES is set, or add
new functions for setting/clearing singular feature bits (so that
F_LOG_ALL can be set/cleared without touching F_PROTOCOL_FEATURES).

Even with such a potential addition to the protocol, we still need this
fix here, because we cannot expect that back-ends will implement this
addition.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
   include/hw/virtio/vhost.h | 10 ++++++++++
   hw/virtio/vhost.c         | 13 +++++++++++++
   2 files changed, 23 insertions(+)

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index a52f273347..2fe02ed5d4 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -90,6 +90,16 @@ struct vhost_dev {
       int vq_index_end;
       /* if non-zero, minimum required value for max_queues */
       int num_queues;
+
+    /*
+     * Whether the virtqueues are supposed to be enabled (via
+     * SET_VRING_ENABLE).  Setting the features (e.g. for
+     * enabling/disabling logging) will disable all virtqueues if
+     * VHOST_USER_F_PROTOCOL_FEATURES is set, so then we need to
+     * re-enable them if this field is set.
+     */
+    bool enable_vqs;
+
       /**
        * vhost feature handling requires matching the feature set
        * offered by a backend which may be a subset of the total
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index a266396576..cbff589efa 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -50,6 +50,8 @@ static unsigned int used_memslots;
   static QLIST_HEAD(, vhost_dev) vhost_devices =
       QLIST_HEAD_INITIALIZER(vhost_devices);

+static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable);
+
   bool vhost_has_free_slot(void)
   {
       unsigned int slots_limit = ~0U;
@@ -899,6 +901,15 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
           }
       }

+    if (dev->enable_vqs) {
+        /*
+         * Setting VHOST_USER_F_PROTOCOL_FEATURES would have disabled all
+         * virtqueues, even if that was not intended; re-enable them if
+         * necessary.
+         */
+        vhost_dev_set_vring_enable(dev, true);
+    }
+
   out:
       return r;
   }
@@ -1896,6 +1907,8 @@ int vhost_dev_get_inflight(struct vhost_dev *dev, 
uint16_t queue_size,

   static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable)
   {
+    hdev->enable_vqs = enable;
+
       if (!hdev->vhost_ops->vhost_set_vring_enable) {
           return 0;
       }

The vhost-user spec doesn't say that VHOST_F_LOG_ALL needs to be toggled
at runtime and I don't think VHOST_USER_SET_PROTOCOL_FEATURES is
intended to be used like that. This issue shows why doing so is a bad
idea.

VHOST_F_LOG_ALL does not need to be toggled to control logging. Logging
is controlled at runtime by the presence of the dirty log
(VHOST_USER_SET_LOG_BASE) and the per-vring logging flag
(VHOST_VRING_F_LOG).

Technically, the spec doesn’t say that SET_LOG_BASE is required.  It says:

“To start/stop logging of data/used ring writes, the front-end may send
messages VHOST_USER_SET_FEATURES with VHOST_F_LOG_ALL and
VHOST_USER_SET_VRING_ADDR with VHOST_VRING_F_LOG in ring’s flags set to
1/0, respectively.”

(So the spec also very much does imply that toggling F_LOG_ALL at
runtime is a valid way to enable/disable logging.  If we were to no
longer do that, we should clarify it there.)

I missed that VHOST_VRING_F_LOG only controls logging used ring writes
while writes to descriptors are always logged when VHOST_F_LOG_ALL is
set. I agree that the spec does require VHOST_F_LOG_ALL to be toggled
at runtime.

What I suggested won't work.


But is there a valid use-case for logging some dirty memory but not all?
I can't understand if this is a feature or a just flaw in specification.




I mean, naturally, logging without a shared memory area to log in to
isn’t much fun, so we could clarify that SET_LOG_BASE is also a
requirement, but it looks to me as if we can’t use SET_LOG_BASE to
disable logging, because it’s supposed to always pass a valid FD (at
least libvhost-user expects this:
https://gitlab.com/qemu-project/qemu/-/blob/master/subprojects/libvhost-user/libvhost-user.c#L1044).

As an aside: I don't understand how logging without an fd is supposed
to work in QEMU's code or in the vhost-user spec. QEMU does not
support that case even though it's written as if shmfd were optional.


So after a cancelled migration, the dirty bitmap SHM will stay around
indefinitely (which is already not great, but if we were to use the
presence of that bitmap as an indicator as to whether we should log or
not, it would be worse).

Yes, continuing to log forever is worse.


So the VRING_F_LOG flag remains.


I suggest permanently enabling VHOST_F_LOG_ALL upon connection when the
the backend supports it. No spec changes are required.

libvhost-user looks like it will work. I didn't look at DPDK/SPDK, but
checking that it works there is important too.

I can’t find VRING_F_LOG in libvhost-user, so what protocol do you mean
exactly that would work in libvhost-user?  Because SET_LOG_BASE won’t
work, as you can’t use it to disable logging.

That's true. There is no way to disable logging.


(For DPDK, I’m not sure.  It looks like it sometimes takes VRING_F_LOG
into account, but I think only when it comes to logging accesses to the
vring specifically, i.e. not DMA to other areas of guest memory?  I
think only places that use vq->log_guest_addr implicitly check it,
others don’t.  So for example,
vhost_log_write_iova()/vhost_log_cache_write_iova() don’t seem to check
VRING_F_LOG, which seem to be the functions generally used for writes to
memory outside of the vrings.  So here, even if VRING_F_LOG is disabled
for all vrings, as long as a log SHM is set, all writes to memory
outside of the immediate vrings seem to cause logging (as long as
F_LOG_ALL is set).)

Hanna


I have CCed people who may be interested in this issue. This is the
first time I've looked at vhost-user logging, so this idea may not work.

Stefan
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