[Qemu-devel] Re: [PATCH] qemu: make virtio-blk PCI compliant by default

[john cooper]

Michael S. Tsirkin wrote:
> commit bf011293faaa7f87e4de83185931e7411b794128 made virtio-blk-pci not
> PCI-compliant, since it makes region 0 (which is an i/o region)
> size > 256, and, since PCI 2.1, i/o regions are limited to 256 bytes size.
> 
> When the ATA serial number feature is off, which is the default,
> make the device spec compliant again, by making region 0 smaller.

I'd hazard this is the cause of the breakage others
encountered -- even when the driver was initialized
but unused.  For some odd reason I hadn't seen nor
been able to reproduce the failure.

The mock-up of an entire ATA IDENTIFY page is really
overkill for what we're trying to accomplish here,
namely passing a 20 byte S/N from qemu to the guest.
However emulating and passing an IDENTIFY page allows
guest apps to interpret the information via an
existing interface, with the guest driver doing nothing
more than transferring the data as opaque.  During
review, other defined fields of the IDENTIFY page were
speculated to be potentially useful thus the entire
512 byte page was passed wholesale.  But it is clearly
more trouble than benefit at this point.  I'll rework
the patch or use an alternate mechanism.

-john

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