Alexander Graf agraf@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:
- Early writeonly register named early_wr in virtio console's config space.
- Host feature flags namely VIRTIO_CONSOLE_F_EARLY_WRITE for telling guest about early-write capability in console device.
Early write mechanism:
- 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.