Virtualization - Jun 2013 - what should a virtio-mmio transport without a backend look like?

On 21 June 2013 19:01, Christopher Covington <cov at codeaurora.org>
wrote:
> Anyhow, I just did a quick experiment with 0-size block devices, and they
seem
> to work for me, although trying to mount the device yields the confusing
> message, "mount: mounting /dev/vda on mount failed: Invalid
argument".
I'm confused -- what's the significance of zero size
block devices?

thanks
-- PMM

On 06/21/2013 02:28 PM, Peter Maydell wrote:
> On 21 June 2013 19:01, Christopher Covington <cov at codeaurora.org>
wrote:
>> Anyhow, I just did a quick experiment with 0-size block devices, and
they seem
>> to work for me, although trying to mount the device yields the
confusing
>> message, "mount: mounting /dev/vda on mount failed: Invalid
argument".
> 
> I'm confused -- what's the significance of zero size
> block devices?
You were proposing to use a valid/existing MagicValue/Version/VendorID with a
special DeviceID that does nothing. I'm saying why not use a valid/existing
MagicValue/Version/VendorID/DeviceID with a special parameter setting, size=0,
that does nothing?

Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by the Linux Foundation.

Il 21/06/2013 20:28, Peter Maydell ha scritto:
> On 21 June 2013 19:01, Christopher Covington <cov at codeaurora.org>
wrote:
>> Anyhow, I just did a quick experiment with 0-size block devices, and
they seem
>> to work for me, although trying to mount the device yields the
confusing
>> message, "mount: mounting /dev/vda on mount failed: Invalid
argument".
> 
> I'm confused -- what's the significance of zero size
> block devices?
I wouldn't be sad if QEMU forbade creation of zero-sized block devices. :)

Paolo

On 21 June 2013 19:45, Christopher Covington <cov at codeaurora.org>
wrote:
> You were proposing to use a valid/existing MagicValue/Version/VendorID with
a
> special DeviceID that does nothing. I'm saying why not use a
valid/existing
> MagicValue/Version/VendorID/DeviceID with a special parameter setting,
size=0,
> that does nothing?
Ah, I see. Well, it sounds from your quoted mount message as if they
do actually do something (ie the kernel finds a block device and
creates a /dev/vda for it), rather than nothing, which seems like
a bad thing. Also, it's mixing a detail of the backend layer (what
a zero-sized disk happens to look like) with the transport layer,
which seems a bit ugly spec-wise.

(Implementation wise I'm not crazy about it either since it would
be way more complicated than saying "no backend? OK, RAZ/WI".)

-- PMM