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Re: [PATCH 3/3] x86/boot: fold branches in video handling code

  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 8 Sep 2021 16:36:09 +0200
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  • Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 08 Sep 2021 14:36:20 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 08.09.2021 15:49, Andrew Cooper wrote:
> On 08/09/2021 14:24, Jan Beulich wrote:
>> Using Jcc to branch around a JMP is necessary only in pre-386 code,
>> where Jcc is limited to disp8. Use the opposite Jcc directly in two
>> places. Since it's adjacent, also convert an ORB to TESTB.
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> This is an easy change reducing the overall trampoline size a little.
>> We're pretty close to needing a 4th page, which I'd prefer to avoid for
>> as long as we can.
> Honestly, it is high time we ditch all of this.
> The interactive video mode menu only appears to have confused users
> who've encountered it, and it is a weird thing to have in the first place.

Iirc that's not something we've invented, but logic we've inherited
from Linux.

> Furthermore, the Multiboot1/2 specs have supported passing
> video/edid/etc information for longer than Xen has been around.

As per patch 1 you can see how successful I was with using mb1 with
grub2 when it came to graphics modes. (Just as a data point: Afaics
the tools configuring grub in our distro still use "multiboot", not
"multiboot2", and hence we'd be hosed there when gfx bits only work
in the latter case.)

> I am not aware of anything we currently obtain via BIOS INT calls which
> we can't rely on the bootloader for.

I'm unaware of replacements for what edd.S does. Which means ...

>  Furthermore, if the bootloader
> can't provide it, we've got 0 chance of our 16bit assembly being able to
> do something useful...

... I don't really agree here.




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