[PATCH v3 1/3] arm64, vmcoreinfo : Append 'PTRS_PER_PGD' to vmcoreinfo

Bhupesh Sharma bhsharma at redhat.com
Mon Jun 10 03:52:55 PDT 2019

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 :)


>> 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
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