Re: [Question] How can I get floating point registers on arm64

[Dave Marchevsky <davemarchevsky@meta.com>]

On 3/8/23 9:20 AM, Grant Seltzer Richman wrote:
> On Tue, Mar 7, 2023 at 7:28 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>>
>> On Thu, Mar 2, 2023 at 11:06 AM Grant Seltzer Richman
>> <grantseltzer@gmail.com> wrote:
>>>
>>> Hi everyone,
>>>
>>> I'm writing a uprobe program that I'm attaching to a function in a go
>>> program on arm64. The function takes a float and as such loads the
>>> parameters via 64-bit floating point registers i.e. `D0`.
>>>
>>> However, the struct pt_regs context that uprobe programs have access
>>> to only has a single set of 31 64-bit registers. These appear to be
>>> the regular general purpose integer registers. My question is - how do
>>> I access the second set of registers? If this question doesn't make
>>> sense, am I misunderstanding how arm64 works?
>>>
>>
>> cc'ing Dave, as he was looking at this problem in the past (in the
>> context of accessing xmm registers, but similar problem).
>>
>> The conclusion was that we'd need to add a new helper (kfunc nowadays)
>> that would do it for BPF program. Few things to consider:
>>
>>   - designing generic enough interface to allow reading various
>> families of registers (FPU, XMM, etc) in some generic way
>>   - consider whether do platform-specific or platform-agnostic
>> interface (both possible)
>>   - and most annoyingly, we'd need to handle kernel potentially
>> modifying FPU state without (yet) restoring it. Dave investigated
>> this, and in some recent kernels it seems like kernel code doesn't
>> necessarily restore FPU state right after it's done with it, and
>> rather sets some special flag to restore FPU state as kernel exits to
>> user-space.
> 
> Thanks for this info Andrii! I think your first couple points are
> manageable but I'm not familiar with FPU context switching. Will read
> up on it, and Dave if you're willing to give some guidance I'd happily
> put in the work to get this helper introduced!
> 

Hi Grant,

I attempted to tackle this in a patchset a while back [0]. Had to abandon it to
focus on other things, please feel free to use it as a starting point.

Happy to elaborate on Andrii's 3rd point above, there's definitely some nuance
there that the series may not explain well. But need a day or so to page it back
in :). Will update this thread with details.

  [0]: https://lore.kernel.org/bpf/20220512074321.2090073-1-davemarchevsky@fb.com/

>>
>> Hopefully Dave can correct me and fill in details.
>>
>>
>>> Thanks so much,
>>> Grant