On Wed, Mar 8, 2023 at 10:54 PM Dave Marchevsky <davemarchevsky@xxxxxxxx> wrote: > > On 3/8/23 9:20 AM, Grant Seltzer Richman wrote: > > On Tue, Mar 7, 2023 at 7:28 PM Andrii Nakryiko > > <andrii.nakryiko@xxxxxxxxx> wrote: > >> > >> On Thu, Mar 2, 2023 at 11:06 AM Grant Seltzer Richman > >> <grantseltzer@xxxxxxxxx> 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. Thanks Dave! I'm going to spend time over the new few days familiarizing myself with this code and will certainly follow up with questions. I see this patchset seems to specifically tackle x86 for now, I'm hoping to additionally get arm64 support as that's the priority for the project I work on. > [0]: https://lore.kernel.org/bpf/20220512074321.2090073-1-davemarchevsky@xxxxxx/ > > >> > >> Hopefully Dave can correct me and fill in details. > >> > >> > >>> Thanks so much, > >>> Grant
