Re: [Xen-devel] [PATCH v2] x86/boot: Fix the boot time relocation calculations

[Julien Grall <julien.grall@xxxxxxx>]

On 14/06/17 11:33, Julien Grall wrote:
> Hi Andrew,
>
> On 06/12/2017 11:45 AM, Andrew Cooper wrote:
> > c/s b28044226e1 "x86: make Xen early boot code relocatable" introduces
> >
> >      mov $sym_offs(__image_base__),%esi
> >
> > to the legacy boot path.  However, this is by definition 0, which
> > means the
> > boot code only functions correctly when Xen is loaded at its preferred
> > physical address (2M at the time of writing).
> >
> > Xen does cope if loaded at an alternative physical address, if the
> > MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR tag is filled in properly.  While
> > recent
> > versions of Grub do fill this in appropriately, tboot does not.  (In
> > fact,
> > tboot loads Xen at the preferred address, but claims a load address of
> > 8M.)
> >
> > Both Multiboot 1 and 2 specify the execution environment as being
> > flat.  As a
> > result, Xen needs no help calculating the proper load address.
> >
> > However, Multiboot specifies %esp as undefined.  Experimentally, using
> > the
> > entry %esp is fine, but this is certainly no guarantee.  Use a
> > temporary stack
> > in the first page of RAM, which is one of the safest areas to clobber.
> >
> > Calculate the load address from %eip alone, and ignore
> > MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR entirely.  This fixes legacy boot
> > under
> > various versions of tboot.
> >
> > Finally, set up the stack as soon as possible, which means the BIOS
> > path has a
> > usable stack for the entirety of its duration.  Use the full available
> > stack
> > size, rather than limiting to an arbitrary 1k.  One side effect is
> > that the
> > MB2/EFI path continues to use the EFI stack until the trampoline is
> > entered.
> >
> > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Tested-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> > ---
> > CC: Jan Beulich <JBeulich@xxxxxxxx>
> > CC: Julien Grall <julien.grall@xxxxxxx>
> > CC: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> > CC: Doug Goldstein <cardoe@xxxxxxxxxx>
> > CC: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> >
> > This is a regression introduced in Xen 4.9, and should therefore be
> > fixed.
> This is touching early-boot code. I would like to wait a least a push
> with this patch on staging before suggesting to push in Xen 4.9.
Release-acked-by: Julien Grall <julien.grall@xxxxxxx>

> Cheers,
--
Julien Grall

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