Re: [PATCH v5 1/6] Revert "xen/x86: bzImage parse kernel_alignment"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.03.2024 15:08, Jason Andryuk wrote:
> On 2024-03-27 04:59, Roger Pau Monné wrote:
>> On Wed, Mar 27, 2024 at 08:22:41AM +0100, Jan Beulich wrote:
>>> On 26.03.2024 22:38, Jason Andryuk wrote:
>>>> A new ELF note will specify the alignment for a relocatable PVH kernel.
>>>> ELF notes are suitable for vmlinux and other ELF files, so this
>>>> Linux-specific bzImage parsing in unnecessary.
>>>>
>>>> This reverts commit c44cac229067faeec8f49247d1cf281723ac2d40.
>>>>
>>>> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
>>>
>>> Since you keep re-sending this: In private discussion Roger has indicated
>>> that, like me, he too would prefer falling back to the ELF data, before
>>> falling back to the arch default (Roger, please correct me if I got it
>>> wrong). That would make it necessary that the change you're proposing to
>>> revert here is actually kept.
>>
>> Sorry, was meaning to reply yesterday but Jason is very fast at
>> sending new version so I'm always one version behind.
> 
> :)
> 
> I was hoping to finish this up and get it in...
> 
>> IMO the order: ELF note, PHDR alignment, arch default should be the
>> preferred one.
>>
>>> Or wait - what you're reverting is taking the alignment out of the
>>> bzImage header. I don't expect the BSDs to use that protocol; aiui that's
>>> entirely Linux-specific.
>>
>> Yeah, I don't have strong opinions in keeping this, we already do
>> bzImage parsing, so we might as well attempt to fetch the alignment
>> from there if correct:
>>
>> ELF note, bzImage kernel_alignment, ELF PHDR alignment, arch default
> 
> I'm not sure how to handle ELF PHDR vs. arch default.  ELF PHDR will 
> always be set, AFAIU.  Should that always be respected, which means we 
> don't need an arch default?

A value of 0 (and 1) is specifically permitted, to indicate no alignment.
We may take 0 to mean default, but what you suggest below is another
plausible approach. Yet another might be to take anything below PAGE_SIZE
as "use default".

> To include arch default, it would be something like this:
> 
>      if ( parms->phys_align != UNSET_ADDR )
>          align = parms->phys_align;
>      else if ( bz_align )
>          align = bz_align;

Why do you include bz again here? Didn't you previously indicate the
header field can't be relied upon? Which is also why, finally, I committed
this revert earlier today.

Jan

>      else if ( elf->palign > PHYS32_RELOC_ALIGN_DEFAULT )
>          align = elf->palign;
>      else
>          align = PHYS32_RELOC_ALIGN_DEFAULT;
> 
> 
>>> I further meanwhile realized that consulting the ELF phdrs may also be
>>> ambiguous, as there may be more than one. I guess it would need to be the
>>> maximum of all of them then.
>>
>> My suggestion (not sure if I mentioned this before) was to use the
>> alignment of the first LOAD PHDR, which is the one that defines the
>> value of the dest_base field used as the image load start address.
>>
>> Using the maximum of all load PHDRs might be safer.
> 
> I'll find the maximum.
> 
> Thanks,
> Jason