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Subject: Re: [virtio-comment] [PATCH RFC] virtio: introduce VIRTIO_F_DEVICE_STOP

• From: Jason Wang <jasowang@redhat.com>
• To: Cornelia Huck <cohuck@redhat.com>
• Date: Tue, 22 Dec 2020 20:51:04 +0800


On 2020/12/22 äå8:14, Cornelia Huck wrote:

On Tue, 22 Dec 2020 15:30:31 +0800
Jason Wang <jasowang@redhat.com> wrote:


On 2020/12/22 äå2:50, Halil Pasic wrote:

On Tue, 22 Dec 2020 10:36:41 +0800
Jason Wang <jasowang@redhat.com> wrote:

On 2020/12/22 äå5:33, Halil Pasic wrote:

On Fri, 18 Dec 2020 12:23:02 +0800
Jason Wang <jasowang@redhat.com> wrote:

This patch introduces a new status bit DEVICE_STOPPED. This will be
used by the driver to stop and resume a device. The main user will be
live migration support for virtio device.

Can you please provide some more background information, or point
me to the appropriate discussion?

I mean AFAIK migration already works without this driver initiated
drain. What is the exact motivation? What about the big picture? I
guess some agent in the guest would have to make the driver issue
the DEVICE_STOP.

This is not necessary if the datapath is done inside qemu and when
migration is initiated by qemu itself.

But it's a must for using virtio-device as a backend for emulated virtio
devices (e.g vhost-vDPA). In this case, qemu needs to stop the device
then it can safely synchronize the state from them.

You say, in this case qemu needs to stop the device, which makes sense
(it also has to do this when the datapath is implemented in qemu), but
AFAIU DEVICE_STOPPED is initiated by the guest and not by qemu. I'm
confused.


It's initiated by Qemu. Guest is unware of live migration.

But isn't setting DEVICE_STOPPED a _driver_ initiated process? That
sounds like "guest" to me.



I think we need clarification on the "driver". Think the following setup:


Guest driver <-> Qemu <-> vhost-vdpa <-> vDPA parent (virtio-pci vDPA driver)


It's the Qemu that initiates the stop, and the request is forwarded to virtio-pci vDPA driver. So the process is transparent to the guest (driver).







I'm still curious about how the different components in the stack
(guest OS, qemu, vdpa-vhost in host kernel, the PCI function) are
supposed to interact.


It works like:

From Qemu point of view, vhost-vDPA is just another type of vhost
backend. Qemu needs to stop virtio (vhost) before it can do migration.
So we require vDPA devices to have the ability of stopping or pausing
its datapath. If the vDPA device is by chance the virtio-PCI device, it
needs an interface for receiving stop/resume command from the driver.

So the devce stop/resume command was sent from Qemu to vhost-VDPA, then
to vDPA parent which could be a virtio-PCI device in this case.

But QEMU implements the _device_, not the driver, doesn't it?




The device is implemented with the co-operation between Qemu and vhost-vDPA. During migration qemu need to stop the virtio-net device, then vhost must be stopped as well.



And IIUC,
vhost-VDPA and the vDPA parent are also on the device side. I feel like
I'm missing something essential here.




Virtio-PCI driver could be a vDPA parent as well in this case. So we need stop the virtio-pci device.







Signed-off-by: Jason Wang <jasowang@redhat.com>
---
content.tex | 26 ++++++++++++++++++++++++--
1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/content.tex b/content.tex
index 61eab41..4392b60 100644
--- a/content.tex
+++ b/content.tex
@@ -47,6 +47,9 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
\item[DRIVER_OK (4)] Indicates that the driver is set up and ready to
drive the device.

+\item[DEVICE_STOPPED (32)] When VIRTIO_F_DEVICE_STOPPED is negotiated,
+  indicates that the device has been stopped by the driver.
+

AFAIU it is not only about indicating stopped, but also requesting to be
stopped.

More importantly, that must not be set immediately, in a sense that the
one side initiates some action by requesting the bit to be set, and the
other side must not set the bit before the action is performed.

Yes.


We also
seem to assume that every device implementation is capable of performing
this trick.

A dedicated feature bit is introduced for this.

This is not about the feature bit, but about the mechanism. But your
subsequent answers explain, that this is nothing unusual, and then
we should be fine.

Is it for hardware devices (e.g. PCI) standard to request an
operation by writing some value into a register, and get feedback bout
a non-completion by reading different value that written,

This is not ununsal in other devices. And in fact, the FEATURES_OK works
like this:

"""

The device MUST NOT offer a feature which requires another feature which
was not offered. The device SHOULD accept any valid subset of features
the driver accepts, otherwise it MUST fail to set the FEATURES_OK device
status bit when the driver writes it.

"""

Thanks for the pointer. I intend to have another look at how FEATURES_OK
works, and how similar this is to DEVICE_STOPPED.


My understanding is that for both of them, driver can try to set the bit
by writing to the status register but it's the device that decide when
to set the bit.

I think there's a difference: For FEATURES_OK, the driver can read back
the status and knows that the feature combination is not accepted if it
is not set. For DEVICE_STOPPED, it does not really know whether the
device has started to stop yet. It only knows that the device is
actually done stopping when DEVICE_STOPPED is set.




The only difference is that device may need sometime to be stopped. FEATURES_OK might not be the best example. Let's see how reset work which is more similar to DEVICE_STOPPED:

"""


After writing 0 to device_status, the driver MUST wait for a read of device_status to return 0 before reinitializing the device.

"""



Do we need a different mechanism for the device to signal the driver
that the device has actually been stopped? If the driver sees the
STOPPED status after reading back, it knows that the device has
acknowledged the request and is now stopping down. If it is not set, it
means that the device has not honoured the request. Same for clearing
it to resume.




I don't see much difference. Device reset doesn't have such notification and driver may simply poll or use a timer.

Thanks




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