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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/10/17 21:08, Daniel Kiper wrote:
> 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.

The meaning of tag, if understood by Grub, is "don't exit boot services
before passing control".

The tag is currently marked as optional, which means Grub is free to
ignore it if it doesn't understand it, resulting in EBS being called
before passing control.

Xen cannot cope with with EBS having been called, so must not be passed
control under those circumstances.

Doug's patch marks it as non-optional which, by that section above,
requires Grub to fail with an error rather than proceeding, if it does
not understand the tag.


By my reading, Doug's patch looks correct.

How does your interpretation of the spec differ?

~Andrew

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