Re: [Xen-devel] [PATCH 3/3] x86/EFI: don't accept 64-bit base relocations on page tables

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/08/16 13:57, Jan Beulich wrote:
>>>> On 19.08.16 at 14:39, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 19/08/16 08:52, Jan Beulich wrote:
>>> Page tables get pre-populated with physical addresses which, due to
>>> living inside the Xen image, will never exceed 32 bits in width. That
>>> in turn results in the tool generating the relocations to produce
>>> 32-bit relocations for them instead of the 64-bit ones needed for
>>> relocating virtual addresses. Hence instead of special casing page
>>> tables in the processing of 64-bit relocations, let's be more rigid
>>> and refuse them (as being indicative of something else having gone
>>> wrong in the build process).
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Is it an ABI requirement to use the minimal available relocation?  It is
>> certainly suboptimal to use a 64bit relocation when a 32bit one would
>> do, but I wouldn't bet that it is unconditional avoided by all toolchains.
> What ABI? The tool in question is one of our own making. And the
> way relocations get generated it's hard to tell those that have to
> be 32-bit (in early boot code and trampoline code) from those that
> may as well be 64-bit ones (in page tables).
>
>> It is currently the case that Xen needs to live below 4GB physical, so
>> from that point of view a 64bit relocation will not be required in the
>> pagetables.
> And even if Xen didn't itself have this requirement, the EFI loader
> would always put us below 4Gb.

Why is this necessarily true?

xen.efi is built as a 64bit PE, not 32bit.

~Andrew

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