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Re: [PATCH 3/3] x86/boot: fold branches in video handling code
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 8 Sep 2021 14:49:04 +0100
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Furthermore, the Multiboot1/2 specs have supported passing
video/edid/etc information for longer than Xen has been around.
I am not aware of anything we currently obtain via BIOS INT calls which
we can't rely on the bootloader for. Furthermore, if the bootloader
can't provide it, we've got 0 chance of our 16bit assembly being able to
do something useful...
Dropping all of that should remove the bsp-only path from the
trampoline, taking it down to just a single page and removing a lot of
fragile code. It gets us closer to being able to support Coreboot
(where the current logic says "not EFI? I can make BIOS INT calls
then", and is wrong).
I know its a larger piece of work, but the wins are substantially larger.
~Andrew
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