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

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.

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