Re: [PATCH] xen/efi: Fix Grub2 boot on arm64

[Luca Fancellu <luca.fancellu@xxxxxxx>]

> On 3 Nov 2021, at 11:28, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 03.11.2021 11:20, Luca Fancellu wrote:
>> 
>> 
>>> On 3 Nov 2021, at 08:20, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>> 
>>> On 02.11.2021 18:12, Luca Fancellu wrote:
>>>>> On 2 Nov 2021, at 14:45, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>> On 02.11.2021 15:05, Luca Fancellu wrote:
>>>>>> The code introduced by commit a1743fc3a9fe9b68c265c45264dddf214fd9b882
>>>>>> ("arm/efi: Use dom0less configuration when using EFI boot") is
>>>>>> introducing a problem to boot Xen using Grub2 on ARM machine using EDK2.
>>>>>> 
>>>>>> The problem comes from the function get_parent_handle(...) that inside
>>>>>> uses the HandleProtocol on loaded_image->DeviceHandle, but the last
>>>>>> is NULL, making Xen stop the UEFI boot.
>>>>> 
>>>>> According to my reading the UEFI spec doesn't (explicitly) allow for
>>>>> this to be NULL. Could you clarify why this is the case? What other
>>>>> information may end up being invalid / absent? Is e.g. read_section()
>>>>> safe to use?
>>>> 
>>>> My test on an arm machine running Grub2 on top of EDK2 showed that
>>>> when Xen is started, the get_parent_handle(…) call was failing and stopping
>>>> the boot because the efi_bs->HandleProtocol(…) was called with the
>>>> loaded_image->DeviceHandle argument NULL and the call was returning
>>>> a EFI_INVALID_PARAMETER.
>>>> So the parent handle can’t be requested and the filesystem can’t be used,
>>>> but any other code that doesn’t use the handle provided by 
>>>> get_parent_handle(…)
>>>> can be used without problem like read_section(...).
>>> 
>>> I understand this. My question was for the reason of ->DeviceHandle
>>> being NULL. IOW I'm wondering whether we're actually talking about a
>>> firmware or GrUB bug, in which case your change is a workaround for
>>> that rather than (primarily) a fix for the earlier Xen change.
>> 
>> The issue was found only when using EDK2+Grub2, no issue when booting
>> directly from EDK2.
>> This is a fix for the regression, because without the EFI changes, Grub2 was
>> booting successfully Xen. Using grub2 to boot Xen on arm is a very common
>> solution so not supporting this anymore could lead to lots of people having
>> issues if they update to Xen 4.16.
> 
> I'm not objecting to addressing the issue. But the description needs
> to make clear where the origin of the problem lies, and afaict that's
> not the earlier Xen change. That one merely uncovered what, according
> to your reply, might then be a GrUB bug. Unless, as said earlier, I
> merely haven't been able to spot provisions in the spec for the field
> in question to be NULL.

Maybe I can rephrase to be more specific from:

The problem comes from the function get_parent_handle(...) that inside
uses the HandleProtocol on loaded_image->DeviceHandle, but the last
is NULL, making Xen stop the UEFI boot.

To:

Despite UEFI specification, EDK2+Grub2 is returning a NULL DeviceHandle,
that is used by efi_bs->HandleProtocol(…) inside get_parent_handle(…),
causing Xen to stop the boot getting an EFI_INVALID_PARAMETER error.

Do you think it can be ok like this?

Cheers,
Luca

> 
> Jan