On 03-Mär 15:56, Alexei Starovoitov wrote:
> On Tue, Mar 03, 2020 at 11:28:12PM +0100, KP Singh wrote:
> > > > +static void align16_branch_target(u8 **pprog)
> > > > +{
> > > > + u8 *target, *prog = *pprog;
> > > > +
> > > > + target = PTR_ALIGN(prog, 16);
> > > > + if (target != prog)
> > > > + emit_nops(&prog, target - prog);
> > > > + if (target != prog)
> > > > + pr_err("calcultion error\n");
> > >
> > > this wasn't in the original code, do you feel like it's more important
> > > to check this and print error?
> > >
> > > also typo: calculation error, but then it's a bit brief and
> > > uninformative message. So I don't know, maybe just drop it?
> >
> > Ah, good catch! this is deinitely not intended to be here.
> > It's a debug artifact and needs to dropped indeed.
>
> That spurious pr_err() caught my attention as well.
> After further analysis there is a bug here.
> The function is missing last line:
> *pprog = prog;
Great catch! Fixed.
> Without it the nop insertion is actually not happenning.
> Nops are being written, but next insns will overwrite them.
> When I noticed it by code review I applied the patches to my tree
> and run the tests and, as expected, all tests passed.
> The existing test_xdp_veth.sh emits the most amount of unaligned
> branches. Since then I've been thinking whether we could add a test
> to catch things like this and couldn't come up with a way to test it
> without burning a lot of code on it. So let's fix it and move on.
> Could you rename this helper? May be emit_align() and pass 16 into it?
Seems reasonable. Done.
> The code is not branch target specific. It's aligning the start
> of the next instruction.
> Also could you add a comment to:
Done. Sending v2 out.
- KP
> align16_branch_target(&prog);
> for (i = 0; i < fmod_ret->nr_progs; i++)
> emit_cond_near_jump(&branches[i], prog, branches[i],
> X86_JNE);
> kfree(branches);
> to say that the loop is updating prior location to jump to aligned
> branch target ?
