Re: [PATCH v2 22/23] x86: make Xen early boot code relocatable

[Daniel Kiper <daniel.kiper@oracle.com>]

On Thu, Aug 27, 2015 at 07:12:38AM -0600, Jan Beulich wrote:
> >>> On 20.07.15 at 16:29, <daniel.kiper@oracle.com> wrote:

[...]

> >          /* Copy bootstrap trampoline to low memory, below 1MB. */
> > -        mov     $sym_phys(trampoline_start),%esi
> > +        lea     sym_offset(trampoline_start)(%ebp),%esi
> >          mov     $trampoline_end - trampoline_start,%ecx
> >          rep     movsb
> >
>
> The latest at this final hunk I'm getting tired (and upset). I'd much
> rather not touch all this code in these fragile ways, and instead
> require Xen to be booted without grub2 on badly written firmware
> like the one on the machine you quote in the description.

Let's start discussion from here. My further work on this patch series
is pointless until we do not agree details in this case.

So, I agree that any software (including firmware) should not use
precious resources (i.e. low memory in that case) if it is not strictly
required. And I do not think so that latest UEFI implementations
on new hardware really need this low memory to survive (at least page
tables could be put anywhere above low memory). However, in case of
UEFI it is a wish of smart people not requirement set by spec. I was
checking UEFI docs a few times but I was not able to find anything
which says that e.g. "...developer MUST allocate memory outside of low
memory ...". Even I have not found any suggestion about that. However,
I must admit that I do not know whole UEFI spec, so, if you know something
which is similar to above please tell me where it is (title, revision,
page, paragraph, etc.). Hence, if there is not strict requirement in
regards to memory allocation in specs we are lost. Of course we can
encourage people to not do nasty things but I do not believe that many
will listen. So, sadly, I suppose that we will see more and more machines
in the wild which are "broken" (well, let's say do not align to good practices).

On the other hand I think that we should not assume that a given memory
region (in our case starting at 1 MiB) is (or will be) available in every
machine. I have not seen anything which says that it is true. We should
stop doing that even if it works right now. I think that it works by
chance and there is a chance that it will stop working one day because
somebody will discover that he or she can put there e.g. device or hole.

Last but not least. I suppose that Xen users and especially distros will
complain when they are not able to use GRUB2 with Xen on every platform
just because somebody (i.e. platform designers, developers, or whatever)
do not accept our beliefs or we require that platform must obey rules
(i.e. memory map requirements) which are specified nowhere.

So, I think that early Xen boot code should be relocatable. However, I agree
that there is a chance that may solution is not perfect. So, I suppose,
taking into account above, we should discuss how to improve it instead
of discussing whether we should do it.

Daniel