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

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Message-ID: <20160429160256.GE28599@redhat.com>
Date:	Fri, 29 Apr 2016 17:02:57 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	jslaby@...e.com, peter@...leysoftware.com,
	andriy.shevchenko@...ux.intel.com, phillip.raffeck@....de,
	anton.wuerfel@....de, yamada.masahiro@...ionext.com,
	matwey@....msu.ru, valentinrothberg@...il.com,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	ddutile@...hat.com
Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs.

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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