linux-kernel - Re: [PATCH 0/2] Early printk support for virtio console devices.

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Date:	Fri, 26 Apr 2013 16:54:01 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Anup Patel <anup.patel@...aro.org>,
	Peter Maydell <peter.maydell@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	Alexander Graf <agraf@...e.de>,
	"linux-kernel@...r.kernel.org List" <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: [PATCH 0/2] Early printk support for virtio console devices.

On 26/04/13 16:03, Arnd Bergmann wrote:
> On Friday 26 April 2013, Anup Patel wrote:
>> I am curious about how smh-based or hypercall-based early prints would
>> be handled in following scenario:
>>
>> "A board is running KVM ARM enabled kernel and linux console on serial
>> port. Now a user remotely connects to the board via telnet/ssh and
>> launches a VM with smh-based or hypercall-based earlyprintk."
>>
>> In the above scenario, will smh-based or hypercall-based earlyprints
>> appear to user on remote shell or not ?
> 
> The implementation of the console device would still be done by KVM,
> as for any other virtual device, so it shows up whereever kvm is
> configured to output the console data.

Actually, at least in the case of smh-based earlyprintk, the output only
appears in the model console, and never reach the rest of the kernel.

And any other non-MMIO approach (like a hypercall) will be very hard to
feed back to the console, as KVM itself has no idea of what a "console"
is. You'd need another side channel to userspace, hoping that it will be
able to merge the multiple streams.

	M.
-- 
Jazz is not dead. It just smells funny...

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