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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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