Re: [Qemu-devel] [PATCH 1/2] pc: make vgabios exit port more useful

[Avi Kivity <avi@redhat.com>]

On 08/08/2011 10:31 PM, Anthony Liguori wrote:
> We've always listened on port 501 for vgabios panic messages.  In the entire
> time I've worked on QEMU, I've never actually seen a vgabios panic message :-)
>
> If we change the semantics of this port a little bit, it makes it possible to
> use it for more interesting use-cases.  I chose this approach instead of adding
> a new I/O port because it avoids having a guest visible change.
>
> This change allows single-byte access to port 501 and also uses the value
> written to construct an exit code.

A little late to review but...

>
> diff --git a/hw/pc.c b/hw/pc.c
> index 1c9d89a..4b07b35 100644
> --- a/hw/pc.c
> +++ b/hw/pc.c
> @@ -549,8 +549,7 @@ static void bochs_bios_write(void *opaque, uint32_t addr, uint32_t val)
>           /* LGPL'ed VGA BIOS messages */
>       case 0x501:
>       case 0x502:
> -        fprintf(stderr, "VGA BIOS panic, line %d\n", val);
> -        exit(1);
> +        exit((val<<  1) | 1);

This code (before the patch) circumvents -no-shutdown.

Shifting val left is surprising.  What's wrong with even exit codes?


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.