From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Wed, 1 May 2013 15:46:22 +0100 Subject: [PATCH 0/2] Early printk support for virtio console devices. In-Reply-To: References: <1366974299-17877-1-git-send-email-pranavkumar@linaro.org> <87obcyc9sh.fsf@rustcorp.com.au> <336C59AA-D5D7-472E-A8DA-EEB413DD6C68@suse.de> <87sj27xxjr.fsf@rustcorp.com.au> Message-ID: <20130501144622.GA24055@mudshark.cambridge.arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, May 01, 2013 at 07:53:49AM +0100, Anup Patel wrote: > On Wed, May 1, 2013 at 7:37 AM, Rusty Russell wrote: > > Alexander Graf writes: > >> There are not device specific registers in > >> virtio-console. Virtio-console lives behind a virtio bus which doesn't > >> know what these registers are. > > > > You're not going to make coherent arguments without reading that actual > > patch we're discussing. And you're going to just waste everyone else's > > time. > > > >> Even if you shove it into config space, > > > > Which is what the patch does... > > > >> it'd be broken because config access happens without intercepts on > >> some platforms. > > > > But people keep assuming it, which is one reason virtio_ccw switched > > from passive lguest-style config to active. Only lguest and the old kvm > > virtio use it, and lguest can change. > > > > An emergency output is a reasonable idea, and this is a reasonable > > implementation. The question is practical: will it be used? Because we > > don't implement reasonable ideas which aren't going to be used. > > We are already using it for Guest bring-up on APM ARM64 board using > KVMTOOL and so far it has been really helpful in debugging KVM ARM64 > guest issues. > > Also, this is a very useful debugging method for Guest bring-up on > other architectures supported by KVM and other Hypervisors that > support VirtIO. FWIW: I've hacked together a small set of patches allowing us to use the x86 ioport peripherals on arm/arm64 in kvmtool. This means you can use the 8250 for earlyprintk on the latter, and even as the main console if you like since it has a device-tree binding. I'll post the patches as an RFC to the kvm list. Will