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

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

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.

> Jan

Frediano