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