Re: [PATCH] Avoid additional relocations in trampoline code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2024 15:56, Frediano Ziglio wrote:
> On Mon, Aug 26, 2024 at 9:21 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 22.08.2024 17:29, Frediano Ziglio wrote:
>>> The trampoline could have "manual" relocation entries (created
>>> by assembly macros and some code to use them) and (in case of PE)
>>> normal executable relocations.
>>> Remove all normal executable ones. In this way we don't have to
>>> worry about applying both correctly (they need proper order
>>> which is hard to spot looking at the code).
>>>
>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
>>
>> I think this wants splitting into one patch replacing sym_offs() and a
>> 2nd one introducing the hand-crafted __XEN_VIRT_START additions (which
>> may want macro-izing). Plus the justification for the change wants
>> extending, to actually explain what the problem is - after all there's
>> no issue anywhere right now.
> 
> Should I explain the time dependency issue with source code?

Time dependency? I guess I don't understand ...

> I suppose I can describe where currently is and why it would be better
> to have it removed (honestly I though I did but reading the commit
> message I didn't).
> Maybe for search reasons it would be better to define 2 macros like
> the following?
> 
> #define phys_addr(sym) sym ## _pa
> #define virt_addr(sym) sym ## _va
> 
> This way, for instance, searching for the "__high_start" word (like
> "grep -rw __high_start") would produce a result.

I assume it's best to keep everything there together, so see below.

>> With the sym_offs() uses gone, I think it would be best if the macro was
>> #undef-ed ahead of the inclusion of trampoline.S. Since x86_64.S uses the
>> macro, that'll require careful re-arrangement of #include order.
>>
> 
> I think you mean including the trampoline after including x86_64.S in
> head.S.

Yes.

>>> --- a/xen/arch/x86/xen.lds.S
>>> +++ b/xen/arch/x86/xen.lds.S
>>> @@ -71,7 +71,12 @@ SECTIONS
>>>    __2M_text_start = .;         /* Start of 2M superpages, mapped RX. */
>>>  #endif
>>>
>>> -  start_pa = ABSOLUTE(start - __XEN_VIRT_START);
>>> +#define DEFINE_PA_ADDRESS(sym) sym ## _pa = ABSOLUTE(sym - 
>>> __XEN_VIRT_START)
>>> +  DEFINE_PA_ADDRESS(start);
>>> +  DEFINE_PA_ADDRESS(saved_magic);
>>> +  DEFINE_PA_ADDRESS(idle_pg_table);
>>> +  DEFINE_PA_ADDRESS(__high_start);
>>> +  DEFINE_PA_ADDRESS(s3_resume);
>>>
>>>    . = __XEN_VIRT_START + XEN_IMG_OFFSET;
>>>    _start = .;
>>
>> For the cases where in assembly code you add __XEN_VIRT_START this is pretty
>> odd: You subtract the value here just to add it back there. Did you consider
>> a more straightforward approach, like introducing absolute xxx_va symbols?
> 
> I didn't consider. Would something like
> 
> #define DEFINE_ABS_ADDRESSES(sym) \
>    sym ## _pa = ABSOLUTE(sym - __XEN_VIRT_START); \
>    sym ## _va = ABSOLUTE(sym)
> 
> make sense? Maybe the _pa and _va suffixes are too similar? Maybe
> _physaddr and _virtaddr? Or use capical letters and macros (as above)
> to avoid possible clashes?

I'd like to ask that we don't introduce symbols we don't actually use. Hence
a single macro defining both is probably not going to be overly helpful. As
to capital letters: I'm struggling with the "(as above)" - I don't see any
use of capital letters in symbol names being generated. But yes, I was going
to suggest to consider _VA and _PA tags, precisely to reduce the risk of
clashes.

Jan