Re: [Xen-devel] [PATCH] x86/boot: fix MB2 header to require EFI BS

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Tue, Oct 24, 2017 at 02:40:41PM -0500, Doug Goldstein wrote:
> The EFI multiboot2 entry point currently requires EFI BootServices to
> not have been exited however the header currently tells the boot
> loader that Xen optionally supports EFI BootServices having been exited.
> With this change Xen properly advertises that EFI must not be exited
> allowing the boot loader to report an error that it cannot boot Xen if
> it is unable to meet its needs.
>
> Signed-off-by: Doug Goldstein <cardoe@xxxxxxxxxx>
> ---
>
> This should likely be applied against Xen 4.9 and Xen 4.10 as well as
> staging. I am trying to get multiboot2 support for iPXE and upstream
> is concerned that leaving EFI BootServices enabled will not be
> compatible with their aims to support Secure Boot. So when I build

Hmmm... What are exact arguments for that? How do they implement e.g.
chain loading then? What about the shim support?

> my iPXE without support for passing on Boot Services, Xen will be
> loaded by iPXE but then it will fall down with "ERR: Bootloader
> shutdown EFI x64 boot services!" implying that this is required. By
> having Xen expose in its header that its required it allows me to
> handle the situation gracefully in iPXE.
>
> To quote the multiboot2 spec exact:
>
> "This tag indicates that payload supports starting without terminating
> boot services."
>
> Unfortunately the spec is a bit vague and how I am reading it is:
> - no tag = exit boot services in the boot loader
> - tag present marked optional = boot loader can or cannot exit boot services
> - tag present marked required = boot loader cannot exit boot services

NACK, please take a look at section 3.1.4, Multiboot2 information request
in Multiboot2 spec. OPTIONAL/REQUIRED has different meaning for the bootloader
than you think.

> In the future I would like to add support to Xen to allow it to run
> without boot services but presently that support isn't there.

I tried that. This is difficult but not impossible. Hmmm... IIRC, some
things are impossible. Please take a look at efi_multiboot2() and you
quickly will know. Though why not try again.

Daniel

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