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Date: Tue, 29 Sep 2009 08:51:55 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: [Qemu-devel] Re: [PATCH 0/2] fix virtio_blk serial pci config
	breakage
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To: john cooper <john.cooper@redhat.com>
Cc: john cooper <john.cooper@third-harmonic.com>, "Michael S. Tsirkin" <mst@redhat.com>, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, jens.axboe@oracle.com, Vadim Rozenfeld <vrozenfe@redhat.com>

john cooper wrote:
> This is a correction to the previous version
> which violated the PCI config space 256 byte
> limitation.
>
> The crux of the feature is simply passing a qemu
> received virtio serial number to/through the guest
> driver and making it available to guest userspace
> via a preexisting interface.
>
> To accomplish this, ATA IDENTIFY data as implemented
> by the HDIO_GET_IDENTITY ioctl is returned to the
> caller for the virtio_blk drive of interest.  Content
> of this identify structure is created by qemu and
> passed wholesale to guest userspace without
> interpretation by the guest driver.
>
> The change this patch implements is passing of the
> identify data through a mapping established by
> PCI BAR #5 rather than the PCI config area, the
> latter of which resulted in the above breakage.
>   

This is a massive layering violation.  The virtio-blk ABI cannot make 
demands of the transport.

The better solution would be to move the entire virtio-pci config space 
to a separate BAR that's an MMIO region.  Then there is no practical 
limit on the size of the config area.

Regards,

Anthony Liguori