linux-kernel - Re: [RFC] arm64: Early printk support for virtio-mmio console devices.

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Message-Id: <7C17A12D-F7C5-48ED-8D2F-22D8A7F289E3@suse.de>
Date:	Thu, 18 Apr 2013 10:51:14 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Peter Maydell <peter.maydell@...aro.org>
Cc:	Marc Zyngier <maz@...terjones.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] arm64: Early printk support for virtio-mmio console devices.

Am 18.04.2013 um 10:30 schrieb Peter Maydell <peter.maydell@...aro.org>:

> On 18 April 2013 07:49, Marc Zyngier <maz@...terjones.org> wrote:
>> If you need an early console, why not simply wire the 8250 emulation in
>> kvmtool to be useable from the MMIO bus? I reckon this would solve your
>> problem in a more elegant way...
> 
> The other approach I thought of would be something involving
> defining a hypercall interface for console I/O, in the same
> way that we have hypercalls for "start cpu"/"stop cpu"/etc.
> Is there any mileage in considering that approach, or is it
> a non-starter?

It's exactly what we had for the s390-virtio machine. Virtio-console as console device plus a hypercall for early printk.

It was a mess.

Trying to inject character output that lands in machine context, where hypercalls get handled, inside of a specific virtio-console device, which owns the char output, is hard. We haven't found any good solution in qemu to layer this properly.

The closest approach to something workable was to create 2 char outputs and mux them together, like you usually would mus monitor and serial. Good luck muxing that one again with the monitor :).

I'd rather spare you guys from going through the same pain.

Alex

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