Re: [Qemu-devel] [PATCH v5 4/4] i386: allow to load initrd below 4G for recent linux

[Li Zhijian <lizhijian@cn.fujitsu.com>]

Hi Michael, Eduardo

On 1/15/19 09:46, Michael S. Tsirkin wrote:
> On Tue, Jan 15, 2019 at 09:35:09AM +0800, Li Zhijian wrote:
>> Hi Eduardo
>>
>>
>> On 1/15/19 01:53, Eduardo Habkost wrote:
>>
>>          +    if (protocol >= 0x20c &&
>>          +        lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
>>          +        /*
>>          +         * Linux has supported initrd up to 4 GB for a very long time (2007,
>>          +         * long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
>>          +         * though it only sets initrd_max to 2 GB to "work around bootloader
>>          +         * bugs". Luckily, QEMU firmware(which does something like bootloader)
>>          +         * has supported this.
>>          +         *
>>          +         * It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
>>          +         * be loaded into any address.
>>          +         *
>>          +         * In addition, initrd_max is uint32_t simply because QEMU doesn't
>>          +         * support the 64-bit boot protocol (specifically the ext_ramdisk_image
>>          +         * field).
>>          +         *
>>          +         * Therefore here just limit initrd_max to UINT32_MAX simply as well.
>>          +         *
>>          +         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
>>          +         * supports up to 4G initrd as well.
>>
>>      I don't understand what exactly this FIXME comment is
>>      documenting.  What exactly needs to be fixed?
>>
>>
>> XLF_CAN_BE_LOADED_ABOVE_4G is one of the indicators, actually as comments said,
>> linux has supported up to 4 GB initrd since linux-2.26(protocol version 0x208).
>>
>>
>> I just want to comment that linux with protocol within [0x208, 0x20c] supports up to 4 GB initrd as well.
>>
>> Is documenting with FIXME appropriate?
>>
>>
>> Thanks
>>
>>
>
> Fixme should say what is missing in the qemu implementation.



thanks for your explanation @Michael
I'd like to update "FIXME" to "NOTE" and move it into git-commit-log if no objection
and it's okay to delete it simply if it confuses others :)

BTW: any other comments for the others

Thanks
Zhijian




> E.g.
>
> /*
>   * Bar 2010 and up can actually be supported using foo.
>   * FIXME: make use of foo to support bar.
>   */
>
>