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Subject: Re: Re: [RESEND PATCH v5 2/5] arm64/crash_core: Export TCR_EL1.T1SZ
 in vmcoreinfo
To:     Bhupesh Sharma <bhsharma@redhat.com>,
        Amit Kachhap <amit.kachhap@arm.com>
Cc:     Mark Rutland <mark.rutland@arm.com>, x86@kernel.org,
        Will Deacon <will@kernel.org>,
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        Kazuhito Hagio <k-hagio@ab.jp.nec.com>,
        James Morse <james.morse@arm.com>,
        Dave Anderson <anderson@redhat.com>,
        bhupesh linux <bhupesh.linux@gmail.com>,
        linuxppc-dev@lists.ozlabs.org,
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        Steve Capper <steve.capper@arm.com>,
        Ray Jui <ray.jui@broadcom.com>,
        Bharat Gooty <bharat.gooty@broadcom.com>
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From:   Scott Branden <scott.branden@broadcom.com>
Message-ID: <d401b003-af3e-c525-ba00-0de48486b7a0@broadcom.com>
Date:   Wed, 29 Apr 2020 16:04:03 -0700
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Hi Bhupesh,

On 2020-02-23 10:25 p.m., Bhupesh Sharma wrote:
> Hi Amit,
>
> On Fri, Feb 21, 2020 at 2:36 PM Amit Kachhap <amit.kachhap@arm.com> wrote:
>> Hi Bhupesh,
>>
>> On 1/13/20 5:44 PM, Bhupesh Sharma wrote:
>>> Hi James,
>>>
>>> On 01/11/2020 12:30 AM, Dave Anderson wrote:
>>>> ----- Original Message -----
>>>>> Hi Bhupesh,
>>>>>
>>>>> On 25/12/2019 19:01, Bhupesh Sharma wrote:
>>>>>> On 12/12/2019 04:02 PM, James Morse wrote:
>>>>>>> On 29/11/2019 19:59, Bhupesh Sharma wrote:
>>>>>>>> vabits_actual variable on arm64 indicates the actual VA space size,
>>>>>>>> and allows a single binary to support both 48-bit and 52-bit VA
>>>>>>>> spaces.
>>>>>>>>
>>>>>>>> If the ARMv8.2-LVA optional feature is present, and we are running
>>>>>>>> with a 64KB page size; then it is possible to use 52-bits of address
>>>>>>>> space for both userspace and kernel addresses. However, any kernel
>>>>>>>> binary that supports 52-bit must also be able to fall back to 48-bit
>>>>>>>> at early boot time if the hardware feature is not present.
>>>>>>>>
>>>>>>>> Since 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) it makes more sense to export the same in
>>>>>>>> vmcoreinfo rather than vabits_actual variable, as the name of the
>>>>>>>> variable can change in future kernel versions, but the architectural
>>>>>>>> constructs like TCR_EL1.T1SZ can be used better to indicate intended
>>>>>>>> specific fields to user-space.
>>>>>>>>
>>>>>>>> User-space utilities like makedumpfile and crash-utility, need to
>>>>>>>> read/write this value from/to vmcoreinfo
>>>>>>> (write?)
>>>>>> Yes, also write so that the vmcoreinfo from an (crashing) arm64
>>>>>> system can
>>>>>> be used for
>>>>>> analysis of the root-cause of panic/crash on say an x86_64 host using
>>>>>> utilities like
>>>>>> crash-utility/gdb.
>>>>> I read this as as "User-space [...] needs to write to vmcoreinfo".
>>> That's correct. But for writing to vmcore dump in the kdump kernel, we
>>> need to read the symbols from the vmcoreinfo in the primary kernel.
>>>
>>>>>>>> for determining if a virtual address lies in the linear map range.
>>>>>>> I think this is a fragile example. The debugger shouldn't need to know
>>>>>>> this.
>>>>>> Well that the current user-space utility design, so I am not sure we
>>>>>> can
>>>>>> tweak that too much.
>>>>>>
>>>>>>>> The user-space computation for determining whether an address lies in
>>>>>>>> the linear map range is the same as we have in kernel-space:
>>>>>>>>
>>>>>>>>      #define __is_lm_address(addr)    (!(((u64)addr) &
>>>>>>>> BIT(vabits_actual -
>>>>>>>>      1)))
>>>>>>> This was changed with 14c127c957c1 ("arm64: mm: Flip kernel VA
>>>>>>> space"). If
>>>>>>> user-space
>>>>>>> tools rely on 'knowing' the kernel memory layout, they must have to
>>>>>>> constantly be fixed
>>>>>>> and updated. This is a poor argument for adding this to something that
>>>>>>> ends up as ABI.
>>>>>> See above. The user-space has to rely on some ABI/guaranteed
>>>>>> hardware-symbols which can be
>>>>>> used for 'determining' the kernel memory layout.
>>>>> I disagree. Everything and anything in the kernel will change. The
>>>>> ABI rules apply to
>>>>> stuff exposed via syscalls and kernel filesystems. It does not apply
>>>>> to kernel internals,
>>>>> like the memory layout we used yesterday. 14c127c957c1 is a case in
>>>>> point.
>>>>>
>>>>> A debugger trying to rely on this sort of thing would have to play
>>>>> catchup whenever it
>>>>> changes.
>>>> Exactly.  That's the whole point.
>>>>
>>>> The crash utility and makedumpfile are not in the same league as other
>>>> user-space tools.
>>>> They have always had to "play catchup" precisely because they depend
>>>> upon kernel internals,
>>>> which constantly change.
>>> I agree with you and DaveA here. Software user-space debuggers are
>>> dependent on kernel internals (which can change from time-to-time) and
>>> will have to play catch-up (which has been the case since the very start).
>>>
>>> Unfortunately we don't have any clear ABI for software debugging tools -
>>> may be something to look for in future.
>>>
>>> A case in point is gdb/kgdb, which still needs to run with KASLR
>>> turned-off (nokaslr) for debugging, as it confuses gdb which resolve
>>> kernel symbol address from symbol table of vmlinux. But we can
>>> work-around the same in makedumpfile/crash by reading the 'kaslr_offset'
>>> value. And I have several users telling me now they cannot use gdb on
>>> KASLR enabled kernel to debug panics, but can makedumpfile + crash
>>> combination to achieve the same.
>>>
>>> So, we should be looking to fix these utilities which are broken since
>>> the 52-bit changes for arm64. Accordingly, I will try to send the v6
>>> soon while incorporating the comments posted on the v5.
>> Any update on the next v6 version. Since this patch series is fixing the
>> current broken kdump so need this series to add some more fields in
>> vmcoreinfo for Pointer Authentication work.
> Sorry for the delay. I was caught up in some other urgent arm64
> user-space issues.
> I am preparing the v6 now and hopefully will be able to post it out
> for review later today.

Did v6 get sent out?

>
> Thanks,
> Bhupesh
>
>
Regards,
Scott