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

[Jason Wang <jasowang@redhat.com>]

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".


> > > 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 ...


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

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.

Thanks


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