Re: [Xen-devel] [PATCH v4 4/5] x86: add multiboot2 protocol support for EFI platforms

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/01/17 14:17, Doug Goldstein wrote:
> diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
> index d423fd8..ac93df0 100644
> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -89,6 +89,13 @@ multiboot2_header_start:
>                     0, /* Number of the lines - no preference. */ \
>                     0  /* Number of bits per pixel - no preference. */
>  
> +        /* Inhibit bootloader from calling ExitBootServices(). */

/* Request that ExitBootServices() not be called. */

This tag doesn't make any guarantees.

> +        .code64
> +
> +__efi64_start:

__mb2_efi64_start:

This entry point is distinct from the PE efi64 entry point in
common/efi/boot.c

> +        cld
> +
> +        /* VGA is not available on EFI platforms. */
> +        movl   $0,vga_text_buffer(%rip)
> +
> +        /* Check for Multiboot2 bootloader. */
> +        cmp     $MULTIBOOT2_BOOTLOADER_MAGIC,%eax
> +        je      .Lefi_multiboot2_proto
> +
> +        /* Jump to not_multiboot after switching CPU to x86_32 mode. */
> +        lea     not_multiboot(%rip),%edi
> +        jmp     x86_32_switch
> +
> +.Lefi_multiboot2_proto:
> +        /* Zero EFI SystemTable and EFI ImageHandle addresses. */
> +        xor     %esi,%esi
> +        xor     %edi,%edi
> +
> +        /* Skip Multiboot2 information fixed part. */
> +        lea     (MB2_fixed_sizeof+MULTIBOOT2_TAG_ALIGN-1)(%rbx),%ecx
> +        and     $~(MULTIBOOT2_TAG_ALIGN-1),%ecx

Are we strictly guaranteed to have the entire image, including multiboot
tags, loaded below 4GB virtual even in the 64bit case? Even on non-EFI
capable grub2's ?

~Andrew

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