Re: [Xen-devel] [PATCH v7 08/14] x86: add multiboot2 protocol support for EFI platforms

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.09.16 at 23:47, <daniel.kiper@xxxxxxxxxx> wrote:
> This way Xen can be loaded on EFI platforms using GRUB2 and
> other boot loaders which support multiboot2 protocol.
> 
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v7 - suggestions/fixes:
>    - do not allocate twice memory for trampoline if we were
>      loaded via multiboot2 protocol on EFI platform,

If you fix bugs not noticed by a reviewer but by yourself, please
drop all acks/R-b-s covering the code in question. And then I'm
afraid I haven't even been able to locate that change - could you
please point out what you did where?

> +        /*
> +         * Initialize BSS (no nasty surprises!).
> +         * It must be done earlier than in BIOS case
> +         * because efi_multiboot2() touches it.
> +         */
> +        lea     .startof.(.bss)(%rip),%edi
> +        mov     $.sizeof.(.bss),%ecx
> +        shr     $3,%ecx
> +        xor     %eax,%eax
> +        rep stosq
> +
> +        pop     %rdi
> +
> +        /*
> +         * efi_multiboot2() is called according to System V AMD64 ABI:
> +         *   - IN: %rdi - EFI ImageHandle, %rsi - EFI SystemTable,
> +         *   - OUT: %rax - highest usable memory address below 1 MiB;
> +         *                 memory above this address is reserved for 
> trampoline;
> +         *                 memory below this address is used for stack and as
> +         *                 a storage for boot data.

How can you validly use memory below this address? (And I'd like to
note that this also changed from v6, and the change to comments
listed in the v7 section and supposedly suggested by me can't cover
this, as I don't recall having asked for such an adjustment.)

Jan


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