Re: [PATCH 3/5] x86: Set xen_phys_start and trampoline_xen_phys_start earlier

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.08.2024 15:50, Frediano Ziglio wrote:
> On Fri, Aug 9, 2024 at 1:59 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 09.08.2024 14:48, Frediano Ziglio wrote:
>>> On Thu, Aug 8, 2024 at 9:25 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 07.08.2024 15:48, Alejandro Vallejo wrote:
>>>>> No reason to wait, if Xen image is loaded by EFI (not multiboot
>>>>> EFI path) these are set in efi_arch_load_addr_check, but
>>>>> not in the multiboot EFI code path.
>>>>> This change makes the 2 code paths more similar and allows
>>>>> the usage of these variables if needed.
>>>>
>>>> I'm afraid I'm struggling with any "similarity" argument here. Imo it
>>>> would be better what, if anything, needs (is going to need) either or
>>>> both of these set earlier. Which isn't to say it's wrong to do early
>>>> what can be done early, just that ...
>>>>
>>>
>>> About similarity is that some part of EFI code expect xen_phys_start
>>> to be initialized so this change make sure that if in the future these
>>> paths are called even for this case they won't break.
>>>
>>>>> --- a/xen/arch/x86/boot/head.S
>>>>> +++ b/xen/arch/x86/boot/head.S
>>>>> @@ -259,6 +259,11 @@ __efi64_mb2_start:
>>>>>          jmp     x86_32_switch
>>>>>
>>>>>  .Lefi_multiboot2_proto:
>>>>> +        /* Save Xen image load base address for later use. */
>>>>> +        lea     __image_base__(%rip),%rsi
>>>>> +        movq    %rsi, xen_phys_start(%rip)
>>>>> +        movl    %esi, trampoline_xen_phys_start(%rip)
>>>>
>>>> ... this path is EFI only if I'm not mistaken, while ...
>>>>
>>>>> @@ -605,10 +610,6 @@ trampoline_setup:
>>>>>           * Called on legacy BIOS and EFI platforms.
>>>>>           */
>>>>>
>>>>> -        /* Save Xen image load base address for later use. */
>>>>> -        mov     %esi, sym_esi(xen_phys_start)
>>>>> -        mov     %esi, sym_esi(trampoline_xen_phys_start)
>>>>
>>>> ... the comment in context is pretty clear about this code also being
>>>> used in the non-EFI case. It is, however, the case that %esi is 0 in
>>>> that case. Yet surely you want to mention this in the description, to
>>>> clarify the correctness of the change.
>>>
>>> Restored this code.
>>
>> Was my analysis wrong then and it's actually needed for some specific
>> case?
> 
> Not clear to what exactly you are referring.
> That later part of code (which was removed) is still needed in case of no-EFI.

Is it? Under what conditions would %esi be non-zero? As indicated by my earlier
reply, I think it would never be. In which case the two stores are pointless.

Jan