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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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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