Re: what should a virtio-mmio transport without a backend look like?

[Christopher Covington <cov@codeaurora.org>]

Hi Peter,

On 06/20/2013 07:08 AM, Pawel Moll wrote:
> On Thu, 2013-06-20 at 11:29 +0100, Peter Maydell wrote:
>> I'm (finally) trying to add virtio-mmio support properly to
>> QEMU now Fred has put all the refactoring foundations in place.
>>
>> 1. One question I've run into is: what should a virtio-mmio transport
>> with no backend look like to the guest OS? The spec as written
>> seems to assume that there's always some backend present.
>> (The idea is that QEMU might just always instantiate say 8
>> mmio transports, and then whether they actually have a
>> blk/net/whatever backend depends on user options).
>>
>> It looks as if the current linux driver insists (if it sees a
>> device tree node) that the MagicValue register at least is
>> correct (otherwise it complains). So one possibility would
>> be "MagicValue/Version/VendorID must read as usual, DeviceID
>> should read as some special "nothing here" value (0?), everything
>> else can RAZ/WI".

Might it be reasonably easy to just not enumerate unused transports in the
device tree or kernel parameters?

Regards,
Christopher

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