From: Eugenio Perez Martin <eperezma@redhat.com>
Sent: Friday, May 27, 2022 3:55 AM

On Fri, May 27, 2022 at 4:26 AM Jason Wang [off-list ref] wrote:

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On Thu, May 26, 2022 at 8:54 PM Parav Pandit [off-list ref] wrote:

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From: Eugenio Pérez <eperezma@redhat.com>
Sent: Thursday, May 26, 2022 8:44 AM

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Implement stop operation for vdpa_sim devices, so vhost-vdpa will
offer

that backend feature and userspace can effectively stop the device.



This is a must before get virtqueue indexes (base) for live
migration,

since the device could modify them after userland gets them. There
are

individual ways to perform that action for some devices

(VHOST_NET_SET_BACKEND, VHOST_VSOCK_SET_RUNNING, ...) but

there

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was no

way to perform it for any vhost device (and, in particular, vhost-vdpa).



After the return of ioctl with stop != 0, the device MUST finish
any

pending operations like in flight requests. It must also preserve
all

the necessary state (the virtqueue vring base plus the possible
device

specific states) that is required for restoring in the future. The

device must not change its configuration after that point.



After the return of ioctl with stop == 0, the device can continue

processing buffers as long as typical conditions are met (vq is
enabled,

DRIVER_OK status bit is enabled, etc).

Just to be clear, we are adding vdpa level new ioctl() that doesn’t map to

any mechanism in the virtio spec.

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We try to provide forward compatibility to VIRTIO_CONFIG_S_STOP. That
means it is expected to implement at least a subset of
VIRTIO_CONFIG_S_STOP.

Appending a link to the proposal, just for reference [1].

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Why can't we use this ioctl() to indicate driver to start/stop the device

instead of driving it through the driver_ok?

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Parav, I'm not sure I follow you here.

By the proposal, the resume of the device is (From qemu POV):
1. To configure all data vqs and cvq (addr, num, ...) 2. To enable only CVQ, not
data vqs 3. To send DRIVER_OK 4. Wait for all buffers of CVQ to be used 5. To
enable all others data vqs (individual ioctl at the moment)

Where can we fit the resume (as "stop(false)") here? If the device is stopped
(as if we send stop(true) before DRIVER_OK), we don't read CVQ first. If we
send it right after (or instead) DRIVER_OK, data buffers can reach data vqs
before configuring RSS.

It doesn’t make sense with currently proposed way of using cvq to replay the config.
Need to continue with currently proposed temporary method that subsequently to be replaced with optimized flow as we discussed.