Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1 to allow up to 4G initrd

[Ingo Molnar]

* H. Peter Anvin <hpa@xxxxxxxxx> wrote:

> On 11/9/18 5:40 AM, Li Zhijian wrote:
> > Just noticed that there is a field xloadflags at recent protocol
> >   60 Protocol 2.12:  (Kernel 3.8) Added the xloadflags field and
> > extension fields
> >   61                 to struct boot_params for loading bzImage and ramdisk
> >   62                 above 4G in 64bit.
> > [snip]
> >  617 Field name:     xloadflags
> >  618 Type:           read
> >  619 Offset/size:    0x236/2
> >  620 Protocol:       2.12+
> >  621
> >  622   This field is a bitmask.
> >  623
> >  624   Bit 0 (read): XLF_KERNEL_64
> >  625         - If 1, this kernel has the legacy 64-bit entry point at
> > 0x200.
> >  626
> >  627   Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
> >  628         - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
> >  629
> > 
> > maybe we can reuse this field and append a new Bit 5
> > XLF_INITRD_MAX_SIZE_4G and increase header version.
> > For the old protocol version 2.12+, if  XLF_CAN_BE_LOADED_ABOVE_4G is
> > set, we can also realize ~4GB initrd is allowed.
> > 
> > bootloader side:
> > if protocol >= 2.15
> >    if XLF_INITRD_LOAD_BELOW_4G
> >       support ~4G initrd
> >    fi
> > else if protocol >=2.12
> >    if XLF_CAN_BE_LOADED_ABOVE_4G
> >     support ~4G initrd
> >    fi
> > fi
> > 
> 
> The two are equivalent.  Obviously you have to load above 4 GB if you
> have more than 4 GB of initrd.  If XLF_CAN_BE_LOADED_ABOVE_4G is not
> set, then you most likely are on a 32-bit kernel and there are more
> fundamental limits (even if you were to load it above the 2 GB mark, you
> would be limited by the size of kernel memory.)
> 
> So, in case you are wondering: the bootloader that broke when setting
> the initrd_max field above 2 GB was, of course, Grub.
> 
> So just use XLF_CAN_BE_LOADED_ABOVE_4G. There is no need for a new flag
> or new field.

That's nice, and that's the best solution!

> Also note that the ext_ramdisk_image and ext_ramdisk_size are part of
> struct boot_params as opposed to struct setup_header, which means that
> they are not supported when entering via the 16-bit BIOS entry point,
> and I am willing to bet that there will be, ahem, "strangeness" if
> entered via the 32-bit entry point if at least the command line is
> loaded above the 4 GB mark; the initrd should be fine, though.
> 
> This is obviosly not an issue in EFI environments, where we enter
> through the EFI handover entry point.
> 
> The main reason these were not added to struct setup_header is that
> there are only 24 bytes left in that header and so space is highly
> precious. One way to deal with that if we really, really need to would
> be to add an initrd/initramfs type of setup_data.

Is there no way to extend that header by making an extended header part 
of the payload?

IIRC that header is small and fixed size to be part of a single sector at 
the very beginning of boot images, but accessing any extended header bits 
from the payload section shouldn't really be an issue for a modern 
bootloader to handle, right?

Such an extended header could use a more modern (self-extending) ABI as 
well.

Thanks,

	Ingo