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, 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 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/