On 30.04.2013, at 02:32, Rusty Russell wrote: > 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.
Fair enough. > >>> 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 quite see the point. The reason early printk works so well for x86 is that you have a UART at a predefined place. > >> 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. How will it live in virtio-console? Virtio-console speaks virtio, not register values. If you put this into the config space you break the s390 virtio model. Alex -- 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/
