Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.
From: Richard W.M. Jones
Date: Fri Apr 29 2016 - 12:03:05 EST
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
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