On Thu, Jan 14, 2021 at 10:37:33PM -0800, Alexei Starovoitov wrote:
> On Thu, Jan 14, 2021 at 1:54 AM Gary Lin <glin@xxxxxxxx> wrote:
> > * pass to emit the final image.
> > */
> > - for (pass = 0; pass < 20 || image; pass++) {
> > - proglen = do_jit(prog, addrs, image, oldproglen, &ctx);
> > + for (pass = 0; pass < MAX_PASSES || image; pass++) {
> > + if (!padding && pass >= PADDING_PASSES)
> > + padding = true;
> > + proglen = do_jit(prog, addrs, image, oldproglen, &ctx, padding);
>
> I'm struggling to reconcile the discussion we had before holidays with
> the discussion you guys had in v2:
>
> >> What is the rationale for the latter when JIT is called again for subprog to fill in relative
> >> call locations?
> >>
> > Hmmmm, my thinking was that we only enable padding for those programs
> > which are already padded before. But, you're right. For the programs
> > converging without padding, enabling padding won't change the final
> > image, so it's safe to always set "padding" to true for the extra pass.
> >
> > Will remove the "padded" flag in v3.
>
> I'm not following why "enabling padding won't change the final image"
> is correct.
> Say the subprog image converges without padding.
> Then for subprog we call JIT again.
> Now extra_pass==true and padding==true.
> The JITed image will be different.
Actually no.
> The test in patch 3 should have caught it, but it didn't,
> because it checks for a subprog that needed padding.
> The extra_pass needs to emit insns exactly in the right spots.
> Otherwise jump targets will be incorrect.
> The saved addrs[] array is crucial.
> If extra_pass emits different things the instruction starts won't align
> to places where addrs[] expects them to be.
>
When calculating padding bytes, if the image already converges, the
emitted instruction size just matches (addrs[i] - addrs[i-1]), so
emit_nops() emits 0 byte, and the image doesn't change.
> So I think the padded flag has to be part of x64_jit_data.
> Please double check my analysis and see why your test keeps working.
> And please add another test that crashes with this v3 and works when
> 'padding' is saved.
> I expected at least some tests in test_progs to be crashing, but
> I've applied patch 1 and run the tests manually and everything passed,
> so I could be missing something or our test coverage for subprogs is too weak.
>
