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Re: [Xen-devel] How to invoke hypercalls

To: antoinet <xen@xxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] How to invoke hypercalls
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Sun, 13 Aug 2006 18:51:44 +0100
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On 13/8/06 6:54 pm, "antoinet" <xen@xxxxxxxxxxx> wrote:

>> Console output doesn't get logged, but you can connect to the console
>> automatically when the domain is created by adding option -c to the end of
>> your 'xm create' command line.
> I also tried the -c switch, but i still can't get any output... Do you
> have any idea why it is so?
> btw: i also tried to do the HYPERCALL_console_io the new way, i.e. by
> calling in the hypercall_page, without success.

D'oh, yes of course. The console_io hypercall is intended only for domain0.
It sends output to whatever console Xen has open (usually just a serial line
after domain0 has booted). Other domains are allowed to use the hypercall
only if Xen is built with verbose=y option. But still that's not useful
unless you have a serial line connected or if you tell Xen to keep the vga
console after domain0 boots (console=vga[keep]).

The usual domU console method is to pick up a console shared page from
start_info structure and write console data to a ring buffer embedded in
that shared page. See Linux's driver for an example, or mini-os's.

Possibly Xen should be able to provide a serial-like interface on top of the
console shared page, which would make initial OS porting a little easier. It
would be something of a layer violation in the system (Xen talking up to
dom0) but we do that already for HVM guests so there's a precedent (albeit
one we'd like to get rid of).

 -- Keir

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