Alexander Graf <ag...@suse.de> writes: > Am 29.04.2013 um 05:09 schrieb Rusty Russell <ru...@rustcorp.com.au>: > >> Alexander Graf <ag...@suse.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, > > If Linux uses it for early console, you won't see any messages from before > the virtio-console driver is initialized, because Linux thinks that it's all > been printed out.
If you can't support it, don't offer the feature. >> 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. > > Why implement it differently from other machines? There are facilities to > call into firmware, so you could use that. There's the special Foundation > model call that you could implement and reuse for this. Sure, for ARM. We *have* a console device. It's the logical place to provide a simple write mechanism. eg. consider bhyve on FreeBSD. > I don't see why anything like this has to live in virtio-mmio. Oh, and it > should default to off. virtio-console, not virtio-mmio. Cheers, Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
