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[Xen-devel] Re: [RFC PATCH 29/35] Add the Xen virtual console driver.

Keir Fraser wrote:

On 22 Mar 2006, at 16:00, Anthony Liguori wrote:

This has always seemed a bit wrong to me and makes a number of things kind of awkward (like a virtual video driver).

It would seem better me to treat this driver as what it really is, a virtual serial device. It adds a little bit of additional work to the userspace tools (they just have to make sure to pass console=ttyS0) but it seems worth it.

That already works pretty much (userspace tools have to pass xencons=ttyS as well as console=ttyS0).
We could also solve the tty[0-9] problem by implementing a proper console driver that could use multiple virtual serial devices if we wanted to go that route.

Yes, that could live alongside this driver.

This is going to come up in the next release if we try to merge the framebuffer stuff.

Another option would be to just emulate a serial driver. The console driver isn't really performance critical. It seems to me that it's a bit unnecessary to even bother paravirtualizing the console device when it's so easy to emulate.

Easy except that Xen can't implement the 'console backend', or at least not easily. The console bits need to end up in management virtual machine's user space. We'd have to do something skanky like the current HVM qemu model.

We could make an exception for console devices and just have a small buffer in the hypervisor for console input/output (similar to the emergency console). The biggest advantage to doing this IMHO is that it makes guest OS's a bit easier to port. The console already is treated as an exception since it is setup in start_info (instead of through XenBus) so this is not that awful I think.


Anthony Liguori

 -- Keir

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