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

[Li Zhijian]

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.
>   */