Re: Freeze with KVM_CLOCK, undocumented dependency VIRTIO_BLK -> VIRTIO_PCI

[Bernhard Schmidt <berni@birkenwald.de>]

Hi,

>> a) when I enable KVM_CLOCK in the kernel configuration the guest system
>> freezes after
>>
>> PCI: Found IRQ 11 for device 0000:00:04.0
>>  vda: vda1 vda2
>> TCP cubic registered
>> NET: Registered protocol family 10
>> NET: Registered protocol family 17
>> Using IPI Shortcut mode
> There are some outstanding patches for KVM_CLOCK that may fix this problem.

Any pointers? I'd love to test and confirm.

>> b) apparently VIRTIO_BLK works only when VIRTIO_PCI and thus PCI is
>> enabled in the kernel. Otherwise the vda* device is not visible and the
>> guest does not find its root filesystem.
>>
>> Is this expected? If yes, is it possible to add the appropriate
>> dependency in the kernel configuration or amend the description
>> accordingly?
> VIRTIO_BLK doesn't depend on VIRTIO_PCI.  VIRTIO_PCI allows you use to 
> virtio devices under KVM.

I don't quite understand. VIRTIO_BLK can be selected in the kernel 
without selecting VIRTIO_PCI, it does not generate any build error but 
the virtual blockdevice simply does appear. Does VIRTIO_BLK have any use 
without VIRTIO_PCI?

> This is all documented FWIW in the Kconfig and in a number of places on 
> the wiki.

Where exactly in Kconfig?

VIRTIO_BLK says nothing about VIRTIO_PCI or PCI, same for VIRTIO_NET. 
Yes, the VIRTIO_PCI Kconfig help says this "provides support for virtio 
based paravirtual device drivers over PCI". But this is far far away 
from the virtio drivers themselves, so one might not immediately notice 
that this is necessary. And if I disabled PCI completely (as a migrating 
Xen user who does not need PCI in domU at all) I won't even see the 
VIRTIO_PCI node, but still be able to select VIRTIO_BLK and _NET (which 
compile just fine but don't work, very confusing).

Bernhard