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

From: Marc Zyngier
Date: Fri Apr 26 2013 - 11:54:11 EST

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.

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

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