linux-kernel - Re: [PATCH v2 03/13] x86: Add macro for 64bit entry startup

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Date:	Tue, 20 Nov 2012 16:20:55 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v2 03/13] x86: Add macro for 64bit entry startup_64

On Tue, Nov 20, 2012 at 1:12 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 11/20/2012 12:55 PM, Yinghai Lu wrote:
>>
>> On Tue, Nov 20, 2012 at 12:44 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>>
>>> I don't see any point in three flags here.  Let's stick to one flag; then
>>> it fits in the existing boot_flags field.
>>
>>
>> the magic AA55
>>
>> boot/header.S:boot_flag:        .word 0xAA55
>>
>> if change that to other value, would it break existing boot loader?
>> aka old boot loader could boot the new bzImage anymore.
>>
>
> Sorry, I meant loadflags.
>
> This is a read flag and so should be low, bit 1 presumably.

yes.

Field name:     loadflags
Type:           modify (obligatory)
Offset/size:    0x211/1
Protocol:       2.00+

  This field is a bitmask.

  Bit 0 (read): LOADED_HIGH
        - If 0, the protected-mode code is loaded at 0x10000.
        - If 1, the protected-mode code is loaded at 0x100000.

  Bit 5 (write): QUIET_FLAG
        - If 0, print early messages.
        - If 1, suppress early messages.
                This requests to the kernel (decompressor and early
                kernel) to not write early messages that require
                accessing the display hardware directly.

  Bit 6 (write): KEEP_SEGMENTS
        Protocol: 2.07+
        - If 0, reload the segment registers in the 32bit entry point.
        - If 1, do not reload the segment registers in the 32bit entry point.
                Assume that %cs %ds %ss %es are all set to flat segments with
                a base of 0 (or the equivalent for their environment).

  Bit 7 (write): CAN_USE_HEAP
        Set this bit to 1 to indicate that the value entered in the
        heap_end_ptr is valid.  If this field is clear, some setup code
        functionality will be disabled.


So will have
  Bit 1 (read): LOADED_ABOVE_4G
  - If 1, code, boot_param, cmdline, ramdisk could be loaded above 4G,

will update the patches accordingly.

>
> On the other hand, the read flags in loadflags has not been modified for a
> very long time, and there is a serious risk that some broken bootloader
> might be doing a full byte comparison.
>
> relocatable_kernel really should have been a flag, but it is now defined as
> a comparison with zero.  As such, and in an effort to minimize the growth of
> struct setup_header (it is limited to little over 128 bytes long) I suggest
> we redefine the 16-byte field at offset 0x236 as a new flags field.  We
> still only need one flag, though.
>
> Backwards compatibility is so much fun.

yes. try to make it ready for future is fun too.

are you going to have pointer for ext_header ?
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