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[Xen-devel] Request to revert superpage adjustments

Sadly, c/s cf393624 "x86: use 2M superpages for text/data/bss mappings"
exposes a bug in all Syslinux variants, including ISOlinux and
PXELinux,  causing a failure to boot.

Xen currently requires its bootloader to decompress it, and place it at
the 1MB physical boundary.  The alignment adjustments changed the size
of the decompressed (post mkelf32) image from 2.2MB to 7.1MB.  These
restrictions should be fixed independently of this exposed bug.  The
physical range between 0x100000 and 0x10fffe is prime clobbering space
for buggy BIOSes once the A20 line has been disabled (see c/s 1ed76797),
and if any reserved memory exists between 1MB and 1MB+sizeof(xen), the
bootloader wont be able to place Xen at its linked address.

Grub and iPXE work perfectly well when booting Xen, which is why this is
now clearly a Syslinux issue (all versions I cared to test, including
4.x and 6.3 are broken).  However, it clobbers any ability for XenServer
to do testing, as we PXEBoot our servers for install.  I expect a lot of
other people will encounter issues once the 4.7 RCs get tested.

Please revert c/s cf393624 and the following change (c/s 53aa3dde) which
depends upon the former, until I can work around the existing
restrictions.  After the restrictions are resolved, the patches can go
back in, but I am fairly sure I will not have time to resolve the issues
in the 4.7 timeframe.


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