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Re: [RFC v2 11/13] vdpa: add vdpa net migration state notifier

From: Si-Wei Liu
Subject: Re: [RFC v2 11/13] vdpa: add vdpa net migration state notifier
Date: Fri, 3 Feb 2023 18:03:56 -0800
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 


On 2/2/2023 7:28 AM, Eugenio Perez Martin wrote:

On Thu, Feb 2, 2023 at 2:53 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:



On 1/12/2023 9:24 AM, Eugenio Pérez wrote:

This allows net to restart the device backend to configure SVQ on it.

Ideally, these changes should not be net specific. However, the vdpa net
backend is the one with enough knowledge to configure everything because
of some reasons:
* Queues might need to be shadowed or not depending on its kind (control
    vs data).
* Queues need to share the same map translations (iova tree).

Because of that it is cleaner to restart the whole net backend and
configure again as expected, similar to how vhost-kernel moves between
userspace and passthrough.

If more kinds of devices need dynamic switching to SVQ we can create a
callback struct like VhostOps and move most of the code there.
VhostOps cannot be reused since all vdpa backend share them, and to
personalize just for networking would be too heavy.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
   net/vhost-vdpa.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++
   1 file changed, 84 insertions(+)

diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
index 5d7ad6e4d7..f38532b1df 100644
--- a/net/vhost-vdpa.c
+++ b/net/vhost-vdpa.c
@@ -26,6 +26,8 @@
   #include <err.h>
   #include "standard-headers/linux/virtio_net.h"
   #include "monitor/monitor.h"
+#include "migration/migration.h"
+#include "migration/misc.h"
   #include "migration/blocker.h"
   #include "hw/virtio/vhost.h"

@@ -33,6 +35,7 @@
   typedef struct VhostVDPAState {
       NetClientState nc;
       struct vhost_vdpa vhost_vdpa;
+    Notifier migration_state;
       Error *migration_blocker;
       VHostNetState *vhost_net;

@@ -243,10 +246,86 @@ static VhostVDPAState 
*vhost_vdpa_net_first_nc_vdpa(VhostVDPAState *s)
       return DO_UPCAST(VhostVDPAState, nc, nc0);
   }

+static void vhost_vdpa_net_log_global_enable(VhostVDPAState *s, bool enable)
+{
+    struct vhost_vdpa *v = &s->vhost_vdpa;
+    VirtIONet *n;
+    VirtIODevice *vdev;
+    int data_queue_pairs, cvq, r;
+    NetClientState *peer;
+
+    /* We are only called on the first data vqs and only if x-svq is not set */
+    if (s->vhost_vdpa.shadow_vqs_enabled == enable) {
+        return;
+    }
+
+    vdev = v->dev->vdev;
+    n = VIRTIO_NET(vdev);
+    if (!n->vhost_started) {
+        return;
+    }
+
+    if (enable) {
+        ioctl(v->device_fd, VHOST_VDPA_SUSPEND);
+    }
+    data_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
+    cvq = virtio_vdev_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ) ?
+                                  n->max_ncs - n->max_queue_pairs : 0;
+    vhost_net_stop(vdev, n->nic->ncs, data_queue_pairs, cvq);
+
+    peer = s->nc.peer;
+    for (int i = 0; i < data_queue_pairs + cvq; i++) {
+        VhostVDPAState *vdpa_state;
+        NetClientState *nc;
+
+        if (i < data_queue_pairs) {
+            nc = qemu_get_peer(peer, i);
+        } else {
+            nc = qemu_get_peer(peer, n->max_queue_pairs);
+        }
+
+        vdpa_state = DO_UPCAST(VhostVDPAState, nc, nc);
+        vdpa_state->vhost_vdpa.shadow_data = enable;
+
+        if (i < data_queue_pairs) {
+            /* Do not override CVQ shadow_vqs_enabled */
+            vdpa_state->vhost_vdpa.shadow_vqs_enabled = enable;
+        }
+    }
+
+    r = vhost_net_start(vdev, n->nic->ncs, data_queue_pairs, cvq);

As the first revision, this method (vhost_net_stop followed by
vhost_net_start) should be fine for software vhost-vdpa backend for e.g.
vp_vdpa and vdpa_sim_net. However, I would like to get your attention
that this method implies substantial blackout time for mode switching on
real hardware - get a full cycle of device reset of getting memory
mappings torn down, unpin & repin same set of pages, and set up new
mapping would take very significant amount of time, especially for a
large VM. Maybe we can do:


Right, I think this is something that deserves optimization in the future.

Note that we must replace the mappings anyway, with all passthrough
queues stopped.
Yes, unmap and remap is needed indeed. I haven't checked, does shadow vq keep mapping to the same GPA where passthrough data virtqueues were associated with across switch (so that the mode switch is transparent to the guest)? For platform IOMMU the mapping and remapping cost is inevitable, though I wonder for the on-chip IOMMU case could it take some fast path to just replace IOVA in place without destroying and setting up all mapping entries, if the same GPA is going to be used for the data rings (copy Eli for his input). 


  This is because SVQ vrings are not in the guest space.
The pin can be skipped though, I think that's a low hand fruit here.
Yes, that's right. For a large VM pining overhead usually overweighs the mapping cost. It would be a great amount of time saving if pin can be skipped. 



If any, we can track guest's IOVA and add SVQ vrings in a hole. If
guest's IOVA layout changes, we can translate it then to a new
location. That way we only need one map operation in the worst case.
I'm omitting the lookup time here, but I still should be worth it.

But as you mention I think it is not worth complicating this series
and we can think about it on top.
Yes, agreed. I'll just let you aware of the need of this optimization for real hardware device. 


  We can start building it on top of
your suggestions for sure.


1) replace reset with the RESUME feature that was just added to the
vhost-vdpa ioctls in kernel

We cannot change vring addresses just with a SUSPEND / RESUME.
I wonder if we can make SUSPEND (via some flag or new backend feature is fine) accept updating internal state like the vring addresses, while defer applying it to the device until RESUME? That way we don't lose a lot of other states that otherwise would need to re-instantiate at large with _F_RING_RESET or device reset. 



We could do it with the VIRTIO_F_RING_RESET feature though. Would it
be advantageous to the device?


2) add new vdpa ioctls to allow iova range rebound to new virtual
address for QEMU's shadow vq or back to device's vq

Actually, if the device supports ASID we can allocate ASID 1 to that
purpose. At this moment only CVQ vrings and control buffers are there
when the device is passthrough.
Yep, we can get SVQ mapping pre-cooked in another ASID before dismantle the mapping for the passthrough VQ. This will help the general IOMMU case. 



But this doesn't solve the problem if we need to send all SVQ
translation to the device on-chip IOMMU, doesn't it? We must clear all
of it and send the new one to the device anyway.


3) use a light-weighted sequence of suspend+rebind+resume to switch mode
on the fly instead of getting through the whole reset+restart cycle


I think this is the same as 1, isn't it?

I mean do all three together: 1,2 in kernel and 3 in QEMU.




I suspect the same idea could even be used to address high live
migration downtime seen on hardware vdpa device. What do you think?


I think this is a great start for sure! Some questions:
a) Is the time on reprogramming on-chip IOMMU comparable to program
regular IOMMU?
I would think this largely depends on the hardware implementation of on-chip IOMMU, the performance characteristics of which is very device specific. Some times driver software implementation and API for on-chip MMU also matters. Which would require vendor specific work to optimize based on the specific use case. 


  If it is the case it should be easier to find vdpa
devices with support for _F_RESET soon.
b) Not to merge on master, but it is possible to add an artificial
delay on vdpa_sim that simulates the properties of the delay of IOMMU?
In that line, have you observed if it is linear with the size of the
memory, with the number of maps, other factors..?
As I said this is very device specific and hard to quantify, but I agree it's a good idea to simulate the delay and measure the effect. For the on-chip MMU device I'm looking, large proportion of the time was spent on software side in allocating a range of memory for hosting mapping entries (don't know how to quantify this part but the allocation time is not a constant nor linear to the size of memory), walking all iotlb entries passed down from vdpa layer and building corresponding memory key objects for a range of pages. For each iotlb entry the time to build memory mapping looks grow linearly with the size of memory. Not sure if there's room to improve, I'll let the owner to clarify. 

Thanks,
-Siwei







Thanks!


Thanks,
-Siwei


+    if (unlikely(r < 0)) {
+        error_report("unable to start vhost net: %s(%d)", g_strerror(-r), -r);
+    }
+}
+
+static void vdpa_net_migration_state_notifier(Notifier *notifier, void *data)
+{
+    MigrationState *migration = data;
+    VhostVDPAState *s = container_of(notifier, VhostVDPAState,
+                                     migration_state);
+
+    switch (migration->state) {
+    case MIGRATION_STATUS_SETUP:
+        vhost_vdpa_net_log_global_enable(s, true);
+        return;
+
+    case MIGRATION_STATUS_CANCELLING:
+    case MIGRATION_STATUS_CANCELLED:
+    case MIGRATION_STATUS_FAILED:
+        vhost_vdpa_net_log_global_enable(s, false);
+        return;
+    };
+}
+
   static void vhost_vdpa_net_data_start_first(VhostVDPAState *s)
   {
       struct vhost_vdpa *v = &s->vhost_vdpa;

+    if (v->feature_log) {
+        add_migration_state_change_notifier(&s->migration_state);
+    }
+
       if (v->shadow_vqs_enabled) {
           v->iova_tree = vhost_iova_tree_new(v->iova_range.first,
                                              v->iova_range.last);
@@ -280,6 +359,10 @@ static void vhost_vdpa_net_client_stop(NetClientState *nc)

       assert(nc->info->type == NET_CLIENT_DRIVER_VHOST_VDPA);

+    if (s->vhost_vdpa.index == 0 && s->vhost_vdpa.feature_log) {
+        remove_migration_state_change_notifier(&s->migration_state);
+    }
+
       dev = s->vhost_vdpa.dev;
       if (dev->vq_index + dev->nvqs == dev->vq_index_end) {
           g_clear_pointer(&s->vhost_vdpa.iova_tree, vhost_iova_tree_delete);
@@ -767,6 +850,7 @@ static NetClientState *net_vhost_vdpa_init(NetClientState 
*peer,
       s->vhost_vdpa.device_fd = vdpa_device_fd;
       s->vhost_vdpa.index = queue_pair_index;
       s->always_svq = svq;
+    s->migration_state.notify = vdpa_net_migration_state_notifier;
       s->vhost_vdpa.shadow_vqs_enabled = svq;
       s->vhost_vdpa.iova_range = iova_range;
       s->vhost_vdpa.shadow_data = svq;
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