[SeaBIOS] Re: [PATCH] Preserve Xen DebugOutputPort

[Kevin O'Connor]

On Tue, Jun 16, 2020 at 10:31:17PM -0400, Jason Andryuk wrote:
> xen_preinit() runs early and changes DebugOutputPort.  qemu_preinit() runs
> soon after.  inb on DebugOutputPort doesn't work on Xen, so the check
> will always fail and DebugOutputPort will be cleared to 0 disabling
> output.
>
> Quick exit the function when running on Xen to preserve the modified
> DebugOutputPort.
> Signed-off-by: Jason Andryuk
>
> src/hw/serialio.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/src/hw/serialio.c b/src/hw/serialio.c
> index 3163344..c6297bc 100644
> --- a/src/hw/serialio.c
> +++ b/src/hw/serialio.c
> @@ -106,6 +106,10 @@ u16 DebugOutputPort VARFSEG = 0x402;
>  void
>  qemu_debug_preinit(void)
>  {
> /* Xen already overrode DebugOutputPort in xen_preinit(). */
> if (runningOnXen())
>    return;

Thanks.  The code looks fine to me.  However, I find the comment
confusing as this function only checks if output is enabled - it
doesn't check the validity of the port itself.  Perhaps a comment like
"Xen doesn't support checking if debug output is active".

-Kevin
> /* Check if the QEMU debug output port is active */
>    if (CONFIG_DEBUG_IO &&
>        inb(GET_GLOBAL(DebugOutputPort)) != QEMU_DEBUGCON_READBACK)