Amit Shah
2022-Nov-23 10:36 UTC
[PATCH v2] virtio_console: Introduce an ID allocator for virtual console numbers
On Wed, 2022-11-23 at 11:17 +0100, C?dric Le Goater wrote:> Hello Amit, > > On 11/22/22 18:03, Amit Shah wrote: > > On Mon, 2022-11-14 at 18:38 +0100, C?dric Le Goater wrote: > > > When a virtio console port is initialized, it is registered as an hvc > > > console using a virtual console number. If a KVM guest is started with > > > multiple virtio console devices, the same vtermno (or virtual console > > > number) can be used to allocate different hvc consoles, which leads to > > > various communication problems later on. > > > > > > This is also reported in debugfs : > > > > > > # grep vtermno /sys/kernel/debug/virtio-ports/* > > > /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 1 > > > /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 1 > > > /sys/kernel/debug/virtio-ports/vport3p1:console_vtermno: 2 > > > /sys/kernel/debug/virtio-ports/vport4p1:console_vtermno: 3 > > > > > > Replace the next_vtermno global with an ID allocator and start the > > > allocation at 1 as it is today. Also recycle IDs when a console port > > > is removed. > > > > When the original virtio_console module was written, it didn't have > > support for multiple ports to be used this way. So the oddity you're > > seeing is left there deliberately: VMMs should not be instantiating > > console ports this way. > > > > I don't know if we should take in this change, but can you walk through > > all combinations of new/old guest and new/old hypervisor and ensure > > nothing's going to break -- and confirm with the spec this is still OK > > to do? It may not be a goal to still ensure launches of a new guest on > > a very old (say) Centos5 guest still works -- but that was the point of > > maintaining backward compat... > > 'next_vtermno' was introduced by d8a02bd58ab6 ("virtio: console: > remove global var") to differentiate the underlying kernel hvc console > associated with each virtio console port. Some drivers, like XEN, > simply use a magic/cookie number for instance. > > This number is not related to the virtio specs. It is not exposed to > QEMU nor the guest (a part from debugfs). It's an internal identifier > related to the implementation in the kernel. I don't understand how > this could break compatibility. The change even keeps the allocated > range the same in case some assumption is made on vtermno 0. Am I > missing something ?No, you're right about this being kernel-internal -- just that it's used with hvc instead of qemu like I mentioned. I think this is the right change; just want to confirm hvc didn't get confused.> > In the virtio console driver case, we could also generate a unique > number from the tuple { virtio device index, virtio console port }. > The ID allocator approach is simpler.I think the bug is that we don't increment the vtermno today in all places that we should; but this patch solves it too - I don't mind adding the extra ida bits.