Re: [PATCH] efi/libstub/arm*: Set default address and size cells values for an empty dtb

[Jeffrey Hugo <jhugo-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 2/7/2017 12:13 PM, Mark Rutland wrote:
> On Tue, Feb 07, 2017 at 12:07:56PM -0700, Jeffrey Hugo wrote:
>> On 2/7/2017 12:01 PM, Mark Rutland wrote:
>>> On Tue, Feb 07, 2017 at 11:54:55AM -0700, Jeffrey Hugo wrote:
>>>> On 2/7/2017 11:12 AM, Ard Biesheuvel wrote:
>>>>> On 7 February 2017 at 17:59, Jeffrey Hugo <jhugo-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org> wrote:
>>>>>> From: Sameer Goel <sgoel-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
>>>>>>
>>>>>> In cases where a device tree is not provided (ie ACPI based system), an
>>>>>> empty fdt is generated by efistub.  Sets the address and size cell values
>>>>>> in a generated fdt to support 64 bit addressing.
>>>>>>
>>>>>> This enables kexec/kdump on Qualcomm Technologies QDF24XX platforms as those
>>>>>> utilities will read the address/size values from the fdt, and such values
>>>>>> may exceed the range provided by the 32 bit default.
>>>>>>
>>>>>
>>>>> As far as I know, those properties are explicitly associated with the
>>>>> 'reg' properties of subordinate nodes. So which nodes are we talking
>>>>> about here? Are we producing an incorrect DT by not setting these? Or
>>>>> is this simply a convenience to work around bugs in the tooling?
>>>>
>>>> I think we are producing an incorrect DT, in some instances.
>>>>
>>>> So we are starting from the same baseline, this is specific to ACPI
>>>> systems, as an ACPI system won't have a DT from the bootloader.  DT
>>>> based systems will already have a DT from the bootloader which is
>>>> assumed to be correct.  On ACPI systems without a DT, efistub
>>>> generates a default one.
>>>>
>>>> That default is assumed to be for a 32-bit system.  The cell width
>>>> defaults to 1, which is 4 bytes.  You cannot represent a 64-bit
>>>> value in that instance.
>>>>
>>>> What happens is that kexec inserts properties into the fdt which
>>>> contain the start address and size on the crash kernel.  On our
>>>> system, the start address is a 64-bit value, and while its not the
>>>> case today, I see no reason why size could not also be a 64-bit
>>>> value.  However the values that are inserted into the fdt are
>>>> governed by the address and size cell values already present in the
>>>> fdt.
>>>>
>>>> Kexec attempts to insert these values in the fdt.  The fdt only
>>>> accepts 32-bit values, so it truncates what is put in.  Then later
>>>> kexec/kdump read the values from the fdt, and get garbage.
>>>
>>> I take it this is specific to the kdump properties?
>>>
>>> I can't immediately see what would matter for the !kdump case.
>>> properties inserted under /chosen are not truncated?
>>
>> The kexec/kdump properties are added under /chosen, therefore yes,
>> properties added under /chosen are truncated, per our observations.
>
> Sorry for the dodgy (and confusing) reply above.
>
> What I was trying to ask was does this *only* affect kdump properties?
> I note that kdump is not yet upstream for arm64.
>
> Or are there regular kexec properties that this affects?
>
> Or !kdump && !kexec properties?
>
> Can you please enumerate the set of properties for which this matters?

"linux,elfcorehdr" and "linux,usable-memory-range"

We are not aware of !kdump && !kexec properties where this is an issue.

>
> Thanks,
> Mark.
>
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-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.