Re: [virtio-dev] Re: [virtio-comment] Re: [PATCH v2 3/3] shared memory: Define mmio registers

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefan Hajnoczi (stefanha@redhat.com) wrote:
> On Wed, Feb 27, 2019 at 02:56:52PM +0100, Cornelia Huck wrote:
> > On Mon, 25 Feb 2019 18:45:06 +0000
> > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > 
> > > * Stefan Hajnoczi (stefanha@redhat.com) wrote:
> > > > On Fri, Feb 22, 2019 at 11:54:33AM +0000, Dr. David Alan Gilbert (git) wrote:  
> > 
> > > > > +    region, as defined by the device for the region selected by
> > > > > +    the \field{SHMId} register.  Reading from a non-existent
> > > > > +    region (i.e. where the ID written to \field{SHMId} is unused)
> > > > > +    results in a length of -1.  
> > > > 
> > > > -1 is used to indicate the absence of a region, does 0 have a meaning?  
> > > 
> > > No, I'd be happy to switch; although this does lead to another question;
> > > what happens on an older virtio-mmio implementation when a device
> > > tries to read these registers to detect if the region exists?
> > 
> > You probably mean that the driver is reading, right? Not sure if we
> > ever specified what happens if a driver interacts with a register that
> > was not specified when the device was written... maybe we need to bump
> > the device version number? But that would break old drivers if they get
> > a 3 but expected a 2.
> 
> I see the rationale for using 0xffffffff since that's what loads produce
> when there is nothing at the memory address.  It would be good to
> document that :).

Is that actually defined that we get 0xffffffff? - in which case
yes I'd add it as a reasoning; but I hadn't realised it was actually
defined anywhere.

> Incrementing the version as Cornelia suggested is cleaner though, if
> existing drivers handle that gracefully.  Or maybe a transport feature
> bit (are they in short supply?).

We do have transport feature bits for other transports don't we, and
they can overlap those.

Dave

> Stefan


--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK