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


On 2021/1/12 äå8:13, Cornelia Huck wrote:

On Tue, 12 Jan 2021 11:27:20 +0800
Jason Wang <jasowang@redhat.com> wrote:


On 2021/1/12 äå2:16, Cornelia Huck wrote:

On Wed, 30 Dec 2020 16:15:47 +0800
Jason Wang <jasowang@redhat.com> wrote:

On 2020/12/29 äå9:35, Halil Pasic wrote:

On Mon, 28 Dec 2020 15:01:57 +0800
Jason Wang <jasowang@redhat.com> wrote:

Some part of the virtio has enforced an asynchronous interface during reset:

For MMIO the spec said:

"""

To stop using the queue the driver MUST write zero (0x0) to this
QueueReady and MUST read the value back to ensure synchronization.

"""

I read the MMIO quote like a single read is sufficient to ensure
synchronization. I.e. it does not require a loop which waits for
the read to yield the expected value.

For PCI it said:

"""

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

"""

Yes, this does sound like a loop. And that's what Linux does. But this
is transport (PCI) specific. On a spec level, a reset is a distinct
operation from setting device status (to 0).

I'm not sure or it looks unclear to me for this point.

E.g the device reset is mentioned in "Device Status Field" belongs to
"Basic Facilities of a Virtio Device". But there's no "Device Reset" in
"Basic Facilities of a Virtio Device".

I think it should be, just to make clear what a driver-initiated reset
of a device actually resets (and that the method for doing so is
transport-specific.)


Then we need some clarifications in the spec. It would be easily imply
that reset is part of device status after reading "Basic Facilities of a
Virtio Device".

I can try to come up with a patch.



Thanks







It just happens to be
mapped to the PCI transport as setting the status to 0.

MMIO did the same. And it makes sense since using a single API to
configure/change device status looks simpler.


For the channel IO transport it is mapped via CCW_CMD_VDEV_RESET while
setting the status is mapped via CCW_CMD_WRITE_STATUS.

Yes, but I think actually there's no limitation if we want to tread 0 as
a reset command for CCW_CMD_WRITE_STATUS.

I'm not a fan of the "driver writes 0 to status to initiate a device
reset" method, but we are stuck with it. I think it's actually not
working well with two other requirements in the spec:

- "The driver MUST NOT clear a device status bit."


Yes, that's kind of conflict but it was how PCI works now ...

Nod.




- "The device MUST initialize device status to 0 upon reset." (This
    suggests to me that a zero status is something set by the device as
    the result of a reset request by the driver, and _not_ set by the
    driver.)


Not a native speaker but we probably need to define "set" and "clear"
mean, E.g:

Clear a bit from driver means write with that bit cleared. Set a bit
from driver means write with that bit set.

Agreed.


Clear a bit from device means read with that be cleared. Set a bit from
device means read with that bit set.

I always saw the status as an actual part of the device. I.e., the
device setting or clearing a bit means that the device is modifying the
status in the device. The driver doing a read will get the status as it
currently is within the device. IOW, the device "controls" the status,
and the driver can submit changes to the status.

[Maybe that's closer to how ccw operates with its commands for reading
or writing the status. Are pci/mmio envisioning the status more like
some kind of shared area?]
My understanding is it's not a shared area, each MMIO read and write will generate a PCI transaction on the bus.
Some developers upstream told me that in the PCIE Endpoint device, the MMIO area could be a shared memory from the view of endpoint device which makes it very hard to make current virtio-pci transport work[1].
[1] https://lore.kernel.org/linux-pci/20190827180114.5979-1-haotian.wang@sifive.com/T/ 





So a question here is how to trigger the device set as we discussed before.



Also, treating CCW_CMD_WRITE_STATUS with 0 as a reset request would be
incompatible with older devices, wouldn't it?


Probably, but we can make both methods work. Note that I'm not
suggesting doing something like this. Just to point out it may work. And
there's something not clear in the spec.

I'm still not convinced that's the way to go. But first, we should be
clear on how the status really is supposed to work.



Right, I meant we had two ways to make the implement aligned with the spec:
1) implement zero write as reset (and maybe keep the old way), so we don't need to modify the spec 2) clarify that transport specific device reset is part of the basic facilities 

Either is fine I think.

Thanks





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