linux-kernel - Re: [RESEND PATCH v5 5/5] Documentation/vmcoreinfo: Add documentation for 'TCR

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Message-ID: <b7d8d603-d9fe-3e18-c754-baf2015acd16@redhat.com>
Date:   Thu, 26 Dec 2019 00:19:35 +0530
From:   Bhupesh Sharma <bhsharma@...hat.com>
To:     James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Cc:     bhupesh.linux@...il.com, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        kexec@...ts.infradead.org, Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Steve Capper <steve.capper@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Anderson <anderson@...hat.com>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>
Subject: Re: [RESEND PATCH v5 5/5] Documentation/vmcoreinfo: Add documentation
 for 'TCR_EL1.T1SZ'

Hi James,

On 12/12/2019 04:02 PM, James Morse wrote:
> Hi Bhupesh,

I am sorry this review mail skipped my attention due to holidays and 
focus on other urgent issues.

> On 29/11/2019 19:59, Bhupesh Sharma wrote:
>> Add documentation for TCR_EL1.T1SZ variable being added to
>> vmcoreinfo.
>>
>> It indicates the size offset of the memory region addressed by TTBR1_EL1
> 
>> and hence can be used for determining the vabits_actual value.
> 
> used for determining random-internal-kernel-variable, that might not exist tomorrow.
> 
> Could you describe how this is useful/necessary if a debugger wants to walk the page
> tables from the core file? I think this is a better argument.
> 
> Wouldn't the documentation be better as part of the patch that adds the export?
> (... unless these have to go via different trees? ..)

Ok, will fix the same in v6 version.

>> diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
>> index 447b64314f56..f9349f9d3345 100644
>> --- a/Documentation/admin-guide/kdump/vmcoreinfo.rst
>> +++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst
>> @@ -398,6 +398,12 @@ KERNELOFFSET
>>   The kernel randomization offset. Used to compute the page offset. If
>>   KASLR is disabled, this value is zero.
>>   
>> +TCR_EL1.T1SZ
>> +------------
>> +
>> +Indicates the size offset of the memory region addressed by TTBR1_EL1
> 
>> +and hence can be used for determining the vabits_actual value.
> 
> 'vabits_actual' may not exist when the next person comes to read this documentation (its
> going to rot really quickly).
> 
> I think the first half of this text is enough to say what this is for. You should include
> words to the effect that its the hardware value that goes with swapper_pg_dir. You may
> want to point readers to the arm-arm for more details on what the value means.

Ok, got it. Fixed this in v6, which should be on its way shortly.

Thanks,
Bhupesh