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


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.




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. 





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. 





We've already had several hardware implementation of virtio-pci devices
from different vendors. I didn't hear any complain about such kind of
design.



   and about the
completion, by reading the same value as written?


After after DEVICE_STOPPED is read from device the driver can assume the
device is stopped.


How is whether the device has already stopped or is still in the middle
of stopping supposed/expected to affect the drivers behavior? Does the
driver actually care?
Yes, in the case of live migration. If device takes too longs time to be stopped. Qemu may simple fail the migration. 







   \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has experienced
     an error from which it can't recover.
   \end{description}
@@ -58,8 +61,9 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
   \ref{sec:General Initialization And Device Operation / Device
   Initialization}.
   The driver MUST NOT clear a
-\field{device status} bit.  If the driver sets the FAILED bit,
-the driver MUST later reset the device before attempting to re-initialize.
+\field{device status} bit other than DEVICE_STOPPED.  If the
+driver sets the FAILED bit, the driver MUST later reset the device
+before attempting to re-initialize.
The driver SHOULD NOT rely on completion of operations of a 
   device if DEVICE_NEEDS_RESET is set.
@@ -70,12 +74,28 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
   recover by issuing a reset.
   \end{note}
+The driver MUST NOT set or clear DEVICE_STOPPED when DRIVER_OK is not 
+set. In order to stop the device, the driver MUST set DEVICE_STOPPED
+first and re-read status to check whether DEVICE_STOPPED is set by the
+device. In order to resume the device, the driver MUST clear
+DEVICE_STOPPED first and read status to ensure whether DEVICE_STOPPED
+is cleared by the device.
+
   \devicenormative{\subsection}{Device Status Field}{Basic Facilities of a Virtio Device / Device Status Field}
   The device MUST initialize \field{device status} to 0 upon reset.
The device MUST NOT consume buffers or send any used buffer 
   notifications to the driver before DRIVER_OK.
+The device MUST ignore DEVICE_STOPPED when DRIVER_OK is not set. 
+
+When driver is trying to set DEVICE_STOPPED, the device MUST not

The when driver trying to set DEVICE_STOPPED is a bit soft as a
duration. For example consider virtio-ccw, at the moment when the driver
issues the ssch to set status, the device still does not know about it.


I need more context on this, if it works like this, when or how can
device know the status has been changed? (E.g how reset or other status
bit is supposed to work?)

I think we have a misunderstanding here. The device  will eventually
learn about the drivers request to write the status, it is just not
instantaneous. It is like when you turn the light switch on, it takes
some amount of time for the current to reach the lightbulb.
I see, so it looks not a problem since device is not required to be stopped immediately if driver write to that bit. 





It looks like a transport limitation if we can't guarantee this. Similar
issue were met in the PCIE Endpoint device, but it can be workaround by
designing a new transport.


I don't think we have a problem with virtio-ccw here.



Agree.





+process new avail requests and MUST complete all requests that is
+currently processing before setting DEVICE_STOPPED.

I would like to have a more precise definition of 'new avail requests'
and 'requests that is currently processing'.


Good point. How about something like:

The device MUST stop reading requests from descriptor area or driver
area and MUST complete all in flight requests before setting DEVICE_STOPPED.

To be 100% accurate, it looks to me we need to mention device
implementation internals or pseudo code. I start with "in flight" but
Stefan wants a more accurate one. Reading the spec I found "in flight"
has been used in:

"""

The driver SHOULD NOT rely on completion of operations of a device if
DEVICE_NEEDS_RESET is set. Note: For example, the driver canât assume
requests in flight will be completed if DEVICE_NEEDS_RESET is set, nor
can it assume that they have not been completed. A good implementation
will try to recover by issuing a reset.

"""


My problem is, that one can look at this from the drivers or from the
devices perspective. From the drivers perspective, every buffer that has
been made available but haven't been marked as used, is essentially a
request in flight.
Not a native speaker, but the sentence starts from "The device" so it should be the perspective of device. 




 From the device perspective, and probably from migration perspective, the
in-flight requests are those that were submitted to some kind of a
backend. I.e. available requests that are just sitting on the queue, but
weren't  submitted to the backend, can sit around, and get submitted
after the migration. It seems that what we want is the following: while
the DEVICE_STOPPED is set, there are no requests in the backend (i.e.
all requests submitted prior have completed, and no more request are
submitted to it).
Yes. but do we need to mention something like "backend" in the spec? It sounds like a implementation detail. 




BTW how about s/stop/drain? I think drain has more of this 'finish
what you are doing' while stop can be 'stop in the middle of it, whatever
you are doing'.
It might be better. But drain may fit more for a queue not a device. We had stuffs other than queue e.g as Stefan pointed out, we should keep config space unchanged (and no config interrupt) after the device is stopped. 





So we are probably fine. Any idea or suggestion are more than welcomed.



I will think about it some more.



Thanks





+
+The device MUST keep the config space unchanged when DEVICE_STOPPED is
+set.

Here you have the set by driver which is actually requesting the stop
operation, and set by device which indicted that the stop operation
was successfully performed by the device.


Exactly.

Thanks


Thank you for all the exlanations!

Regards,
Halil

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