linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Date:   Mon, 25 Oct 2021 14:42:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Mon, Oct 25, 2021 at 03:44:24PM +0200, Maciej Fijalkowski wrote:
> On Fri, Oct 22, 2021 at 08:22:35AM -0700, Alexei Starovoitov wrote:
> > On Fri, Oct 22, 2021 at 4:33 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Thu, Oct 21, 2021 at 04:42:12PM -0700, Alexei Starovoitov wrote:
> > >
> > > > Ahh. Right. It's potentially a different offset for every prog.
> > > > Let's put it into struct jit_context then.
> > >
> > > Something like this...
> > 
> > Yep. Looks nice and clean to me.
> > 
> > > -       poke->tailcall_bypass = image + (addr - poke_off - X86_PATCH_SIZE);
> > > +       poke->tailcall_bypass = ip + (prog - start);
> > >         poke->adj_off = X86_TAIL_CALL_OFFSET;
> > > -       poke->tailcall_target = image + (addr - X86_PATCH_SIZE);
> > > +       poke->tailcall_target = ip + ctx->tail_call_direct_label - X86_PATCH_SIZE;
> > 
> > This part looks correct too, but this is Daniel's magic.
> > He'll probably take a look next week when he comes back from PTO.
> > I don't recall which test exercises this tailcall poking logic.
> > It's only used with dynamic updates to prog_array.
> > insmod test_bpf.ko and test_verifier won't go down this path.
> 
> Please run ./test_progs -t tailcalls from tools/testing/selftests/bpf and
> make sure that all of the tests are passing in there, especially the
> tailcall_bpf2bpf* subset.

Yeah, so nothing from that selftests crud wants to work for me; also I
*really* dislike how vmtest.sh as found there tramples all over my
source dir without asking.

Note that even when eventually supplied with O=builddir (confusingly in
front of it), it doesn't want to work and bails with lots of -ENOSPC
warnings (I double checked, my disks are nowhere near full). (and this
is after installing some horrendous python rst crap because clearly
running a test needs to build documentation :/)

I've spend hours on that, I'm not sinking more time into it. If you want
me to run that crap, fix it first.