Re: [PATCH V7 v7] virtio: introduce SUSPEND bit in device status

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 03, 2024 at 05:05:40PM +0800, Zhu Lingshan wrote:
> >> Are there any similar
> >> constraining for other states like RESET or DRIVER_OK? Don't assume any
> >> other states transitions are faster than SUSPEND.
> > DRIVER_OK does not suffer from it because it is async notification.
> > A device may start slow after DRIVER_OK.
> > SUSPEND operation cannot rely on such async behavior.
> The driver can set suspend and re-read & wait. This is a common routine in the driver.

I don't buy all this talk about special machinery, some 10 nor gates,
the cost is in the noise - we are talking about cards with several
ARM processors on chip.

But I don't like it that looking at the registers, one does not know
the device state. Hidden state is bad for debuggability.
We have 4 states:
suspending->suspended->resuming->resumed
so we need a register with at least 2 bits.

we could steal 2 bits from status but it seems a bit much.



> > RESET also suffers from similar inefficiencies.
> > But that is because it is inherited from the past.
> >
> > Here a new functionality is being proposed and it has a chance for efficient device implementation.
> > Therefore the request is to improve it.
> sure, as long as we confirm this new register apply for all device status transitions, not only for SUSPEND.