[virtio-dev] Re: [PATCH v3 1/2] content: Explain better the status clearing bits

[Cornelia Huck <cohuck@redhat.com>]

On Thu, Nov 11 2021, Eugenio Pérez <eperezma@redhat.com> wrote:

> The spec tells that "The driver MUST NOT clear a device status bit", but
> a device using PCI transport reset a virtio device writing 0 to device

I think MMIO uses that mechanism as well?

> status. In some way, that is to clear all its bits.
>
> Instead of add an exception, tell explicitely the status bits that
> the driver cannot clear anytime in a normal operation, so conformant
> device and drivers keeps being conformant.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  content.tex | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/content.tex b/content.tex
> index 5d112af..2aa3006 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -60,9 +60,10 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
>  initialization sequence specified in
>  \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.
> +The driver MUST NOT clear ACKNOWLEDGE, DRIVER, DRIVER_OK, FEATURES_OK or
> +DEVICE_NEEDS_RESET bits of \field{device status}, except if resetting the whole
> +device.  If the driver sets the FAILED bit, the driver MUST later reset the
> +device before attempting to re-initialize.

I think we need to distinguish "driver wants to clear a status bit" from
"driver is initiating a reset, and that transport implements that by
writing 0 to the device status". So, what about

"The driver MUST NOT clear a \field{device status} bit, except when
setting \field{device status} to 0 as a transport-specific way to
intitiate a reset."

If we introduce driver-clearable bits later, we can simply make that
"The driver MUST NOT clear a \field{device status} bit other than
NEW_BIT, ..."


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