Re: [PATCH 1/2] hvc_xen: add earlycon support

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

> I could do the same here by dropping the if (!xen_pv_domain()) check
> above, but then if somebody specifies earlyprintk=xenboot on a non-Xen
> environment, I expect Linux would crash.

Nah, you made it "Work" with:
commit eb5ef07151ba3c3cb4bcef0c8f146ff1115eaa55
Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Date:   Fri Jan 27 18:31:36 2012 +0000

    hvc_xen: support PV on HVM consoles

But this patch:

commit 04b772d2b819f0dda2163e3193fa7cd447a6245c

xen/hvc: If we use xen_raw_printk let it also work on HVM guests.
    
    The xen_raw_printk works great for debugging purposes. We use
    for PV guests and we can also use it for HVM guests.
    
    However, for HVM guests we have a fallback of using the 0xe9
    port in case the hypervisor does not support an HVM guest of
    using the console_io hypercall. As such lets use 0xe9 during
    early bootup, and once the hyper-page is setup and if the
    console_io hypercall is supported - use that. Otherwise we
    will fallback to using the 0xe9 after early bootup.
    
    We also alter the return value for dom0_write_console to return
    an error value instead of zero. The HVC API has been supporting
    returning error values for quite some time.
    
    P.S.
    To use (and to see the output in the Xen ring buffer) one has to build
    the hypervisor with 'debug=y'.

Should make it possible for HVM guests to actually work with HVM x86 guests
if tweaked.