Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

Richard W.M. Jones
2016-04-29T16:03:00Z

On Fri, Apr 29, 2016 at 08:54:13AM -0700, Greg KH wrote:
> You are trying to take a generalized kernel and somehow "know" about the
> hardware ahead of time it is going to run on. That seems like two
> conflicting requirements, don't you agree?

We would have the 8250 serial port in any kernel. Even if Fedora
kernel maintainers allowed us to have specialized kernels for each
purpose, I would use the simple ISA serial port here because it allows
us to capture debug messages very early in the boot. Alternatives
like virtio-console don't allow that.

The kernel does know what hardware it's running on - via the CPUID
hypervisor leaf. It's also possible for us to tell the kernel about
the hardware using the command line, ACPI[*], DT, etc. I'd really
like to tell the kernel this is a 16550A, not broken, you don't need
to spend time testing that.

There is prior art here: no_timer_check & lpj=..

Rich.

[*] Although ACPI is really slow, adding another 190ms, and for this
reason I have disabled it for now, but not investigated why it's so
slow.

--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org