On Wed, Aug 28, 2024 at 3:47 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
> > It can be je/je too, no?
>
> Yes. It is possible.
>
> >
> > so 128 - 4 instead of 128 - 3 ?
>
> You probably mean "127 - 4 instead of 127 - 3" since
> the maximum value is 127.
Yes, of course :)
> I checked 127 - 4 = 0x7c and indeed we should. See below examples:
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX je 0x291
> 213: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 28d: XX XX XX XX XX XX je 0x212
> 293: bf 03 00 00 00 mov edi,0x3
>
> =>
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX XX XX XX XX je 0x297 (0x293 - 0x213)
> 217: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 291: XX XX je 0x217 (0x217 - 0x293)
> 293: bf 03 00 00 00 mov edi,0x3
>
> =>
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX je 0x28f (0x293 - 0x217)
> 213: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 28d: XX XX je 0x213 (0x213 - 0x293) // -0x80 allowed
> 293: bf 03 00 00 00 mov edi,0x3
>
> =>
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX XX XX XX XX je 0x28f (0x293 - 0x213)
> 217: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 291: XX XX je 0x217 (0x217 - 0x293)
> 293: bf 03 00 00 00 mov edi,0x3
>
> =>
> ...
>
>
> Here 0x293 - 0x217 = 0x7c
How did you craft such a test?
Can we add it as a selftest somehow?
>
> >
> >> +static bool is_imm8_cond_offset(int value)
> >> +{
> >> + return value <= 124 && value >= -128;
> > the other side needs the same treatment, no ?
>
> good question. From my understanding, the non-convergence in the
> above needs both forward and backport conditions. The solution we
> are using is based on putting a limitation on forward conditions
> w.r.t. jit code gen.
>
> Another solution is actually to put a limitation on backward
> conditions. For example, let us say the above is_imm8_cond_offset()
> has
> return value <= 127 && value > -124
>
> See below example:
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX je 0x291
> 213: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 28d: XX XX XX XX XX XX je 0x212
> 293: bf 03 00 00 00 mov edi,0x3
>
> =>
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX XX XX XX XX je 0x297 (0x293 - 0x213)
> 217: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 291: XX XX XX XX XX XX je 0x21b (0x217 - 0x293)
> 297: bf 03 00 00 00 mov edi,0x3
>
> =>
>
> 20e: 48 85 ff test rdi,rdi
> 211: XX XX XX XX XX XX je 0x297 (0x297 - 0x217)
> 217: 48 8b 77 00 mov rsi,QWORD PTR [rdi+0x0]
> ...
> 291: XX XX XX XX XX XX je 0x217 (0x217 - 0x297)
> 297: bf 03 00 00 00 mov edi,0x3
>
> converged here.
>
> So I think we do not need to limit both sides. One side should be enough.
I see and agree when both sides are je/je.
What if the earlier one is a jmp ?
Then we can hit:
if (nops != 0 && nops != 3) {
pr_err("unexpected jump padding: %d bytes\n",
nops);
?
So one side of "jmp_cond padding" and the same side in "jump padding"
needs to do it?
