linux-kernel - question about reserve_bios

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Message-ID: <f8f1e21b-197b-78fc-dc03-606e98c7d783@cn.fujitsu.com>
Date:   Fri, 4 Jan 2019 15:35:03 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
CC:     <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: question about reserve_bios_regions

Hi,

  I am looking into these code via find_trampoline_placement, and I have
some questions still not clear after lots of investigation, hope to get
some hints.

1. How is the value of BIOS_START_MIN(128k) determined? I don't find any
clue about it until now.

2. BIOS_START_MAX is define as 0x9f000U(636k), but both the comments say
640k, is that a quirk or just typo?  I thought the 4k difference is the
quirk, but later I guess the real quirk lies in latter judgment of :

  if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)

I have checked out commit edce21216a8: ("x86/boot: Reorganize and clean
up the BIOS area reservation code"), still no clue.

-- 
Sincerely,
Cao jin