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

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Message-ID: <87obcyc9sh.fsf@rustcorp.com.au>
Date:	Mon, 29 Apr 2013 12:39:34 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Alexander Graf <agraf@...e.de>,
	Pranavkumar Sawargaonkar <pranavkumar@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	"linux-kernel\@vger.kernel.org List" <linux-kernel@...r.kernel.org>,
	kvmarm@...ts.cs.columbia.edu,
	Peter Maydell <peter.maydell@...aro.org>
Subject: Re: [PATCH 0/2] Early printk support for virtio console devices.

Alexander Graf <agraf@...e.de> writes:
> On 26.04.2013, at 13:04, Pranavkumar Sawargaonkar wrote:
>
>> This patch-set implements early printk support for virtio console devices without using any hypercalls.
>> 
>> The current virtio early printk code in kernel expects that hypervisor will provide some mechanism generally a hypercall to support early printk. This patch-set does not break existing hypercall based early print support.
>> 
>> This implementation adds:
>> 1. Early writeonly register named early_wr in virtio console's config space.
>> 2. Host feature flags namely VIRTIO_CONSOLE_F_EARLY_WRITE for telling guest about early-write capability in console device.
>> 
>> Early write mechanism:
>> 1. When a guest wants to out some character, it has to simply write the character to early_wr register in config space of virtio console device.
>
> I won't nack this patch set, but I'll definitely express that I'm not happy with it.
>
> MMIO registers are handled by a different layer than the virtio console itself. After the virtio refactoring in QEMU, they will be completely separate drivers. So we'll be in a similar mess with early printk as we are on the s390-virtio machine, where early printk is done through hypercalls and thus we can't directly link it to the console output.
>
> I still don't see what the issue is with just implementing a small irq-less virtio driver for early printk.

Well, this shouldn't be mmio-specific, but I kind of get what you mean.

I consider this misnamed: it's an emergency write facility.  Linux may
use it for an early console, but it's also useful for bringup and to
give a method of emitting errors like "the console ring is corrupt".

A valid implementation may well be to only offer it with some magic
qemu developer-only commandline and dump it to stdout.

So I think it has use, but less so if QEMU isn't ever going to implement
it.

Cheers,
Rusty.

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