From: Bhupesh Sharma <bhsharma@redhat.com>
To: James Morse <james.morse@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
Kazuhito Hagio <k-hagio@ab.jp.nec.com>,
Steve Capper <Steve.Capper@arm.com>,
kexec@lists.infradead.org, Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org, Dave Anderson <anderson@redhat.com>,
bhupesh.linux@gmail.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 1/3] arm64, vmcoreinfo : Append 'PTRS_PER_PGD' to vmcoreinfo
Date: Mon, 10 Jun 2019 16:22:55 +0530 [thread overview]
Message-ID: <3ab2ca1a-95de-cecf-f590-1e2d00bb644b@redhat.com> (raw)
In-Reply-To: <2a4af3a0-1342-fdd2-1cfd-e37abb99d8bd@arm.com>
Hi James,
On 06/07/2019 08:41 PM, James Morse wrote:
> Hi Bhupesh,
>
> (sorry for the delay on this)
No problem.
> On 04/05/2019 13:53, Bhupesh Sharma wrote:
>> On 04/03/2019 11:24 PM, Bhupesh Sharma wrote:
>>> On 04/02/2019 10:56 PM, James Morse wrote:
>>>> Yes the kernel code is going to move around, this is why the information we expose via
>>>> vmcoreinfo needs to be thought through: something we would always need, regardless of how
>>>> the kernel implements it.
>>>>
>
>>>> Pointer-auth changes all this again, as we may prefer to use the bits for pointer-auth in
>>>> one TTB or the other. PTRS_PER_PGD may show the 52bit value in this case, but neither TTBR
>>>> is mapping 52bits of VA.
>>>>
>>>>
>>>>> So far, I have generally come across discussions where the following variations of the
>>>>> address spaces have been proposed/requested:
>>>>> - 48bit kernel VA + 48-bit User VA,
>>>>> - 48-bit kernel VA + 52-bit User VA,
>>>>
>>>> + 52bit kernel, because there is excessive quantities of memory, and the kernel maps it
>>>> all, but 48-bit user, because it never maps all the memory, and we prefer the bits for
>>>> pointer-auth.
>>>>
>>>>> - 52-bit kernel VA + 52-bit User VA.
>>>>
>>>> And... all four may happen with the same built image. I don't see how you can tell these
>>>> cases apart with the one (build-time-constant!) PTRS_PER_PGD value.
>>>>
>>>> I'm sure some of these cases are hypothetical, but by considering it all now, we can avoid
>>>> three more kernel:vmcoreinfo updates, and three more fix-user-space-to-use-the-new-value.
>>>
>>> Agree.
>>>
>>>> I think you probably do need PTRS_PER_PGD, as this is the one value the mm is using to
>>>> generate page tables. I'm pretty sure you also need T0SZ and T1SZ to know if that's
>>>> actually in use, or the kernel is bodging round it with an offset.
>>>
>>> Sure, I am open to suggestions (as I realize that we need an additional VA_BITS_ACTUAL
>>> variable export'ed for 52-bit kernel VA changes).
>
> (stepping back a bit:)
>
> I'm against exposing arch-specific #ifdefs that correspond to how we've configured the
> arch code's interactions with mm. These are all moving targets, we can't have any of it
> become ABI.
Sure, I understand your concerns.
> I have a straw-man for this: What is the value of PTE_FILE_MAX_BITS on your system?
> I have no idea what this value is or means, an afternoon's archaeology would be needed(!).
> This is something that made sense for one kernel version, a better idea came along, and it
> was replaced. If we'd exposed this to user-space, we'd have to generate a value, even if
> it doesn't mean anything. Exposing VA_BITS_ACTUAL is the same.
>
> (Keep an eye out for when we change the kernel memory map, and any second-guessing based
> on VA_BITS turns out to be wrong)
>
>
> What we do have are the hardware properties. The kernel can't change these.
>
>
>>> Also how do we standardize reading T0SZ and T1SZ in user-space. Do you propose I make an
>>> enhancement in the cpu-feature-registers interface (see [1]) or the HWCAPS interface
>>> (see [2]) for the same?
>
> cpufeature won't help you if you've already panic()d and only have the vmcore file. This
> stuff needs to go in vmcoreinfo.
>
> As long as there is a description of how userspace uses these values, I think adding
> key/values for TCR_EL1.TxSZ to the vmcoreinfo is a sensible way out of this. You probably
> need TTBR1_EL1.BADDR too. (it should be specific fields, to prevent 'new uses' becoming ABI)
>
> This tells you how the hardware was configured, and covers any combination of TxSZ tricks
> we play, and whether those address bits are used for VA, or ptrauth for TTBR0 or TTRB1.
Fair enough. Let me try and experiment with this suggestion a bit and I
will come back with a RFC patch/patchset by this weekend. Hopefully, it
will cover all the weird PA/VA bit combinations we are handling in arm64
distros these days :)
Thanks,
Bhupesh
>> Any comments on the above points? At the moment we have to carry these fixes in the
>> distribution kernels and I would like to have these fixed in upstream kernel itself.
>
>
> Thanks,
>
> James
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>
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next prev parent reply other threads:[~2019-06-10 10:53 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-20 5:09 [PATCH v3 0/3] Append new variables to vmcoreinfo (PTRS_PER_PGD for arm64 and MAX_PHYSMEM_BITS for all archs) Bhupesh Sharma
2019-03-20 5:09 ` [PATCH v3 1/3] arm64, vmcoreinfo : Append 'PTRS_PER_PGD' to vmcoreinfo Bhupesh Sharma
2019-03-26 16:36 ` James Morse
2019-03-27 16:07 ` Kazuhito Hagio
2019-04-02 17:27 ` James Morse
2019-04-05 20:23 ` Kazuhito Hagio
2019-03-28 11:42 ` Bhupesh Sharma
2019-04-02 17:26 ` James Morse
2019-04-03 17:54 ` Bhupesh Sharma
2019-05-04 12:53 ` Bhupesh Sharma
2019-06-07 15:11 ` James Morse
2019-06-10 10:52 ` Bhupesh Sharma [this message]
2019-03-20 5:09 ` [PATCH v3 2/3] crash_core, vmcoreinfo: Append 'MAX_PHYSMEM_BITS' " Bhupesh Sharma
2019-03-20 5:09 ` [PATCH v3 3/3] Documentation/vmcoreinfo: Add documentation for 'MAX_PHYSMEM_BITS' Bhupesh Sharma
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