linux-kernel - Re: [PATCH] pstore/ram: Add support for dynamically allocated ramoops memory regions

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Message-ID: <bd531778-a587-e4d0-e360-432208f064ea@kernel.org>
Date:   Tue, 4 Jul 2023 08:07:09 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Mukesh Ojha <quic_mojha@...cinc.com>,
        Elliot Berman <quic_eberman@...cinc.com>,
        Kees Cook <kees@...nel.org>,
        Isaac Manjarres <isaacmanjarres@...gle.com>,
        Kees Cook <keescook@...omium.org>
Cc:     John Stultz <jstultz@...gle.com>, Tony Luck <tony.luck@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        kernel-team@...roid.com, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, Trilok Soni <quic_tsoni@...cinc.com>,
        Satya Durga Srinivasu Prabhala <quic_satyap@...cinc.com>
Subject: Re: [PATCH] pstore/ram: Add support for dynamically allocated ramoops
 memory regions

On 26/06/2023 19:34, Mukesh Ojha wrote:
> 
> 
> On 6/23/2023 1:21 AM, Elliot Berman wrote:
>>
>>
>> On 6/22/2023 10:58 AM, Kees Cook wrote:
>>> On June 22, 2023 10:26:35 AM PDT, Isaac Manjarres 
>>> <isaacmanjarres@...gle.com> wrote:
>>>> On Wed, Jun 21, 2023 at 10:15:45PM -0700, Kees Cook wrote:
>>>>> On Wed, Jun 21, 2023 at 09:47:26PM -0700, John Stultz wrote:
>>>>>>> The reserved memory region for ramoops is assumed to be at a fixed
>>>>>>> and known location when read from the devicetree. This is not 
>>>>>>> desirable
>>>>>>> in environments where it is preferred for the region to be 
>>>>>>> dynamically
>>>>>>> allocated early during boot (i.e. the memory region is defined with
>>>>>>> the "alloc-ranges" property instead of the "reg" property).
>>>>>>>
>>>>>>
>>>>>> Thanks for sending this out, Isaac!
>>>>>>
>>>>>> Apologies, I've forgotten much of the details around dt bindings here,
>>>>>> so forgive my questions:
>>>>>> If the memory is dynamically allocated from a specific range, is it
>>>>>> guaranteed to be consistently the same address boot to boot?
>>>>>>
>>>>>>> Since ramoops regions are part of the reserved-memory devicetree
>>>>>>> node, they exist in the reserved_mem array. This means that the
>>>>>>> of_reserved_mem_lookup() function can be used to retrieve the
>>>>>>> reserved_mem structure for the ramoops region, and that structure
>>>>>>> contains the base and size of the region, even if it has been
>>>>>>> dynamically allocated.
>>>>>>
>>>>>> I think this is answering my question above, but it's a little opaque,
>>>>>> so I'm not sure.
>>>>>
>>>>> Yeah, I had exactly the same question: will this be the same
>>>>> boot-to-boot?
>>>>
>>>> Hi Kees,
>>>>
>>>> Thank you for taking a look at this patch and for your review! When the
>>>> alloc-ranges property is used to describe a memory region, the memory
>>>> region will always be allocated within that range, but it's not
>>>> guaranteed to be allocated at the same base address across reboots.
>>>>
>>>> I had proposed re-wording the end of the commit message in my response
>>>> to John as follows:
>>>>
>>>> "...and that structure contains the address of the base of the region
>>>> that was allocated at boot anywhere within the range specified by the
>>>> "alloc-ranges" devicetree property."
>>>>
>>>> Does that clarify things better?
>>>
>>> I am probably misunderstanding something still, but it it varies from 
>>> boot to boot, what utility is there for pstore if it changes? I.e. the 
>>> things written during the last boot would then no longer accessible at 
>>> the next boot? E.g.:
>>>
>>> Boot 1.
>>> Get address Foo.
>>> Crash, write to Foo.
>>> Boot 2.
>>> Get address Bar, different from Foo.
>>> Nothing found at Bar, so nothing populated in pstorefs; crash report 
>>> from Boot 1 unavailable.
>>>
>>> I feel like there is something I don't understand about the Foo/Bar 
>>> addresses in my example.
>>>
>>
>> I believe this is being added to support the QCOM SoC minidump feature. 
>> Mukesh has posted it on the mailing lists here:
>>
>> https://lore.kernel.org/all/1683133352-10046-1-git-send-email-quic_mojha@quicinc.com/
>>
>> https://lore.kernel.org/all/1683133352-10046-10-git-send-email-quic_mojha@quicinc.com/
>>
>> Mukesh, could you comment whether this patch is wanted for us in the 
>> version you have posted? It looks like maybe not based on the commit 
>> text in patch #9.
> 
> No, this is no needed after patch #9 .
> 
> I have tried multiple attempt already to get this patch in
> 
> https://lore.kernel.org/lkml/1675330081-15029-2-git-send-email-quic_mojha@quicinc.com/
> 
> later tried the approach of patch #9 along with minidump series..

For all dynamic reserved-memory-ramoops thingy, I think Rob made clear
statement:

"I don't think dynamic ramoops location should be defined in DT."

https://lore.kernel.org/lkml/CAL_JsqKV6EEpsDNdj1BTN6nODb=hsHkzsdkCzzWwnTrygn0K-A@mail.gmail.com/

Please do not send three versions of the same patch hoping one will
sneak in.

Best regards,
Krzysztof