[virtio-comment] Re: [PATCH v3 1/2] virtio-net: Describe dev cfg fields read only

[Cornelia Huck <cohuck@redhat.com>]

On Wed, Feb 22 2023, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Feb 22, 2023 at 01:07:07PM +0100, Cornelia Huck wrote:
>> On Wed, Feb 22 2023, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> 
>> > On Wed, Feb 22, 2023 at 10:01:24AM +0100, Cornelia Huck wrote:
>> >> On Tue, Feb 21 2023, "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> >> 
>> >> > On Tue, Feb 21, 2023 at 05:59:52PM +0000, Parav Pandit wrote:
>> >> >> 
>> >> >> > From: Michael S. Tsirkin <mst@redhat.com>
>> >> >> > Sent: Tuesday, February 21, 2023 12:52 PM
>> >> >> > 
>> >> >> > On Tue, Feb 21, 2023 at 05:50:09PM +0000, Parav Pandit wrote:
>> >> >> > > Hence, it should be mentioned as read-only fields, so when the driver writes
>> >> >> > something to read-only fields, it can be considered as undefined behavior on
>> >> >> > such fields.
>> >> >> > >
>> >> >> > 
>> >> >> > In the description not in the normative statements. normative sections just tell
>> >> >> > driver what it must and must not do, in the standard RFC terms.
>> >> >> > 
>> >> >> Got it.
>> >> >> I will shift them as read-only in the description section.
>> >> >> And normative in the device and driver section.
>> >> >> Device section:
>> >> >> Any writes to config space fields is ignored by the device, because these are read-only fields for the driver.
>> >> >
>> >> > writes is plural so "are ignored"
>> >> >
>> >> > but more importantly use rfc terms in normative sections.
>> >> 
>> >> I don't think you need to talk about "read-only" in the normative
>> >> sections (that belongs to the descriptive sections.) I'd use
>> >> 
>> >> "The device MUST ignore any writes to config space fields by the
>> >> driver."
>> >
>> > Hmm. Is this something we previously required for read only fields?
>> 
>> So, better make it SHOULD?
>> 
>> (The only alternative to ignoring I see is breaking the device, and I
>> think ignoring is preferable.)
>
> We can just skip adding a new requirement completely - we'll never get
> there with a compliant driver.  This is what we do e.g. for MMIO.
> Why not?

That would be fine with me as well.

> This has an advantage as this allows backing config with regular RAM.
> Also I feel that since it always said "read only for driver" then
> this implies a restriction on driver not the device.

Indeed. The normative statement below should be enough to make that
"read-only for the driver" thing obvious.

>
>> >
>> >
>> >> >
>> >> >> 
>> >> >> Driver section:
>> >> >> Driver must not write to read-only fields.
>> >> 
>> >> "The driver MUST NOT write to any config space field."


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