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

Jeffrey Hugo
2017-02-07T19:07:56Z

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.

>
>> By changing the defaults to 2 (the proposed change), 64-bit values
>> can be inserted into the fdt, so the values we put in don't get
>> truncated, and thus kexec/kdump read the correct thing when they
>> need the values.
>>
>> I don't see how the tools could be fixed - fdt is truncating the
>> values, and the generated fdt is already "static" at the point the
>> tools run. We haven't had luck changing the cell size at the point
>> the tools run. Additionally, this seems to be an issue for
>> everything using the fdt - pushing the problem to every tool instead
>> of fixing once at the top seems like playing a game of whack-a-mole.
>>
>> Does that clarify that issue for you? Obviously the commit text
>> needs some work, but I'd like to get on the same page first.
>
> If you could update the commit message to explicitly mention the
> properties being inserted for which this matters, this generally sounds
> fine to me.
>
> Please Cc me on v2.
>
> 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.