Re: [PULL 18/23] accel/tcg: re-factor non-RAM execution code

[Peter Maydell]

On Thu, 15 Apr 2021 at 14:18, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Thu, 18 Feb 2021 at 09:47, Alex Bennée <alex.bennee@linaro.org> wrote:
> >
> > There is no real need to use CF_NOCACHE here. As long as the TB isn't
> > linked to other TBs or included in the QHT or jump cache then it will
> > only get executed once.
> >
> > Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> > Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> > Message-Id: <20210213130325.14781-19-alex.bennee@linaro.org>
>
> Hi; I've just noticed that this commit seems to break the case of:
>  * execution of code not from a RAM block
>  * when icount is enabled
>  * and an instruction is an IO insn that triggers io-recompile
>
> because:
>
> > @@ -2097,6 +2086,17 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
> >          tb_reset_jump(tb, 1);
> >      }
> >
> > +    /*
> > +     * If the TB is not associated with a physical RAM page then
> > +     * it must be a temporary one-insn TB, and we have nothing to do
> > +     * except fill in the page_addr[] fields. Return early before
> > +     * attempting to link to other TBs or add to the lookup table.
> > +     */
> > +    if (phys_pc == -1) {
> > +        tb->page_addr[0] = tb->page_addr[1] = -1;
> > +        return tb;
> > +    }
>
> we used to fall through here, which meant we called
> tcg_tb_insert(tb). No we no longer do. That's bad, because
> cpu_io_recompile() does:
>
>     tb = tcg_tb_lookup(retaddr);
>     if (!tb) {
>         cpu_abort(cpu, "cpu_io_recompile: could not find TB for pc=%p",
>                   (void *)retaddr);
>     }
>
> and since it can no longer find the TB, QEMU aborts.

Adding the tcg_tb_insert() call to the early exit path:

diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
index ba6ab09790e..6014285e4dc 100644
--- a/accel/tcg/translate-all.c
+++ b/accel/tcg/translate-all.c
@@ -2081,6 +2081,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
      */
     if (phys_pc == -1) {
         tb->page_addr[0] = tb->page_addr[1] = -1;
+        tcg_tb_insert(tb);
         return tb;
     }

seems to fix my test case, but I don't know enough about the new
design here to know if that has undesirable side effects.

-- PMM