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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.08.16 at 14:32, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 31/08/16 13:28, Jan Beulich wrote:
>>>>> On 30.08.16 at 18:35, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> 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.
>> The file format doesn't matter here. And we'd have other problems
>> to solve if we got loaded above 4Gb.
> 
> Right, but that doesn't prohibit the generation of 64bit relocations in
> the pagetables.

How that? As already said, we produce the relocations by a tool of
our own, and that tool together with how the inputs passed to it get
built preclude 64-bit relocations in page tables (or else our image
size would need to come close to or exceed 4Gb).

Jan


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