Re: [PATCH v4 5/5] x86/PVH: Support relocatable dom0 kernels

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.03.2024 14:24, Jason Andryuk wrote:
> On 2024-03-26 03:50, Jan Beulich wrote:
>> On 25.03.2024 21:45, Jason Andryuk wrote:
>>> @@ -227,6 +239,27 @@ elf_errorstatus elf_xen_parse_note(struct elf_binary 
>>> *elf,
>>>       case XEN_ELFNOTE_PHYS32_ENTRY:
>>>           parms->phys_entry = val;
>>>           break;
>>> +
>>> +    case XEN_ELFNOTE_PHYS32_RELOC:
>>> +        parms->phys_reloc = true;
>>> +
>>> +        if ( descsz >= 4 )
>>> +        {
>>> +            parms->phys_max = elf_note_numeric_array(elf, note, 4, 0);
>>> +            elf_msg(elf, " = max: %#"PRIx32, parms->phys_max);
>>
>> As indicated before, I consider the = here a little odd.
> 
> I retained = for consistency with other notes:
> ELF: note: PHYS32_RELOC = max: 0x40000000 min: 0x1000000 align: 0x200000
> ELF: note: PHYS32_ENTRY = 0x1000000
> ELF: note: GUEST_OS = "linux"
> 
> I guess whitespace and labels makes it clear, so I'll drop the '='.
> 
>>> +        }
>>> +        if ( descsz >= 8 )
>>> +        {
>>> +            parms->phys_min = elf_note_numeric_array(elf, note, 4, 1);
>>> +            elf_msg(elf, " min: %#"PRIx32, parms->phys_min);
>>> +        }
>>> +        if ( descsz >= 12 )
>>> +        {
>>> +            parms->phys_align = elf_note_numeric_array(elf, note, 4, 2);
>>> +            elf_msg(elf, " align: %#"PRIx32, parms->phys_align);
>>> +        }
>>
>> I'd like us to reconsider this ordering: I'm inclined to say that MAX isn't
>> the most likely one a guest may find a need to use. Instead I'd expect both
>> MIN and ALIGN wanting to be given higher priority; what I'm less certain
>> about is the ordering between the two. To keep MIN and MAX adjacent, how
>> about ALIGN, MIN, MAX?
> 
> ALIGN, MIN, MAX works for me.
> 
> On the Linux side, I'm expecting them all to be set:
> ALIGN = CONFIG_PHYSICAL_ALIGN
> MIN = LOAD_PHYSICAL_ADDR
> MAX = KERNEL_IMAGE_SIZE
> 
> You need enough identity page tables to cover up to MAX. 
> LOAD_PHYSICAL_ADDR is used as a minimum, so requesting placement above 
> MIN makes sense to me.

Hmm, setting MIN like this means moving down is precluded. Why would it
not be possible to move a kernel to lower than the default of 16M, when
CONFIG_PHYSICAL_START can be as low as 0? (In fact, I doubt 0 would work
if chosen, but 2M surely does work, as I build some of my Dom0 kernels
that way.)

MAX, otoh, I guess really wants setting as you say, for KERNEL_IMAGE_SIZE
actually being commented upon as mis-named. Just that it now really wants
to be KERNEL_IMAGE_SIZE-1.

Jan