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Description of problem:
start a guest with 26 virtio disks and with -boot menu=on ,after VM start press F12, I found there are only 7 virtio disks lists in boot menu

Version-Release number of selected component (if applicable):
# rpm -q qemu-kvm
qemu-kvm-0.12.1.2-2.160.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Start a guest with 26 virtio disks and with -boot menu=on (CLI will attached)
2.after guest start press F12

Actual results:
only can find 7 virtio disks lists in boot menu (referring to screendump)

Expected results:
all the virtio disks should list in boot menu

Additional info:

Created attachment 500325 [details]
CommandLine

Created attachment 500326 [details]
Screendump

Bios environment has limited resources and each virtio queue take enormous amount of memory (at least 5404 byte for each queue). Bios fails to initialize more then 7 virtio disks because there is no enough memory for all of them.

Bios always has only one request in flight to virtio block, so virtio queue shouldn't be so big (1 element should be qnough, we have 128 now), but unfortunately, in current virtio implementation host device decides what virtio queue length should be and a guest has no way to negotiate smaller queue size. The proper fix would be to have queue size negotiable between a host and a guest.

Another fix would be to have only one vq for all virtio devices and reinitialize it on each disk access. This is hackish and may slow down BIOS access to virtio disk though.

As a temporary solution we can also enlarge amount of memory available to BIOS, but then by specifying more virtio disks it will be possible to overflow that too.

So the question is do we really need to support such big number of virtio disks?

I think Gleb is right and it is not requirement.