Re: [Xen-devel] Request to revert superpage adjustments

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.03.16 at 14:21, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 11/03/16 12:06, Jan Beulich wrote:
>>>>> On 11.03.16 at 10:22, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> 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.
>> I'm kind of hesitant to do a wholesale revert, for two reasons:
>>
>> 1) The change would still be useful for xen.efi, which is relocatable
>> already anyway.
> 
> The latter change strictly depends on .init having 2M alignment, so
> needs to go either way.  As for making a split, I am already out of time
> which is why I opted for the plain revert.

How about I try to find time to put together a partial revert
(hopefully early) next week?

>> 2) I cannot currently see how you mean to address the issue:
>> Even if you made our binary decompress itself, that would only
>> paper over the issue, and (based on your description) only until
>> even the compressed image exceeds a certain size.
> 
> Compressed, Xen is currently 700k and Linux weighs in at ~4MB, which is
> half the size of the uncompressed Xen with superpages.

Well, okay, there's some headroom (albeit my xen.gz binaries all are
above 900k).

>>  Nor would
>> that deal with avoiding reserved regions not too far above 1Mb,
>> since at best the main payload can be relocatable (but certainly
>> not the binary seen by the boot loader, as at least multiboot1
>> doesn't support anything like that).
> 
> The only reason Xen sits at the 1MB boundary is because of its ELF header.
> 
> A plain binary with a multiboot header has no such restriction, although
> we flag an interested to have 4k alignment using the multiboot flags. 
> There is no technical limitation causing Xen to be linked to run at 1MB;
> its just the way its alway been.  There is nothing preventing the entry
> point becoming properly relocatable.

Without proper container format, how would the boot loader
know where to place the binary, or how to relocate it if it doesn't
get placed at its linked address? The only alternatives I see in
grub1 are a.out and a kludge of a.out, both of which - at the first
glance - also don't appear to do any relocation. And for the Linux
variant, as said, it doesn't look like it's compatible with us needing
multiple modules.

Jan


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