Re: [PATCH 2/3] xen/ppc: Relocate kernel to physical address 0 on boot

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.08.2023 01:37, Shawn Anastasio wrote:
> On 7/31/23 10:46 AM, Jan Beulich wrote:
>> On 29.07.2023 00:21, Shawn Anastasio wrote:
>>> +    /* If we're at the correct address, skip copy */
>>> +    cmpld   %r1, %r12
>>> +    beq     .L_correct_address
>>
>> Can this ever be the case, especially with the MMU-off behavior you
>> describe in the comment above? Wouldn't you need to ignore the top
>> four bits in the comparison?
> 
> It will always be the case after the code jumps to XEN_VIRT_START after
> the copy takes place.

Well, of course.

> I could have it jump past the copy loop entirely,
> but then I'd need to duplicate the TOC setup.

I don't think I understand this part of your reply: .L_correct_address
_is_ past the copy loop.

>>> +    /* Copy bytes until _end */
>>> +    LOAD_REG_ADDR(%r11, _end)
>>> +    addi    %r1, %r1, -8
>>> +    li      %r13, -8
>>> +.L_copy_xen:
>>> +    ldu     %r10, 8(%r1)
>>> +    stdu    %r10, 8(%r13)
>>> +    cmpld   %r1, %r11
>>> +    blt     .L_copy_xen
>>> +
>>> +    /* Jump to XEN_VIRT_START */
>>> +    mtctr   %r12
>>> +    bctr
>>> +.L_correct_address:
>>
>> Can the two regions potentially overlap? Looking at the ELF header
>> it's not clear to me what guarantees there are that this can't
>> happen.
> 
> As I understand it, any bootloader that placed the kernel at a low
> enough address for this to be an issue wouldn't be able to boot Linux or
> FreeBSD, so in practice it's a safe bet that this won't be the case.

Fair enough then.

Jan