Re: [PATCH v3 3/4] target/i386: Make translator stop before the end of a page

[Richard Henderson]

On 8/8/22 10:10, Ilya Leoshkevich wrote:
> Right now translator stops right *after* the end of a page, which
> breaks reporting of fault locations when the last instruction of a
> multi-insn translation block crosses a page boundary.
>
> An implementation, like the one arm and s390x have, would require an
> i386 length disassembler, which is burdensome to maintain. Another
> alternative would be to single-step at the end of a guest page, but
> this may come with a performance impact.
>
> Fix by snapshotting disassembly state and restoring it after we figure
> out we crossed a page boundary. This includes rolling back cc_op
> updates and emitted ops. Even though i386 is the only architecture that
> does rollback, split it into common and architecture-dependent parts to
> improve readability.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   accel/tcg/translator.c      |  8 ++++++++
>   include/exec/translator.h   |  3 +++
>   target/i386/tcg/translate.c | 21 ++++++++++++++++++++-
>   3 files changed, 31 insertions(+), 1 deletion(-)
>
> diff --git a/accel/tcg/translator.c b/accel/tcg/translator.c
> index fe7af9b943..2c4dd09df8 100644
> --- a/accel/tcg/translator.c
> +++ b/accel/tcg/translator.c
> @@ -56,6 +56,7 @@ void translator_loop(const TranslatorOps *ops, 
> DisasContextBase *db,
>   {
>       uint32_t cflags = tb_cflags(tb);
>       bool plugin_enabled;
> +    TCGOp *last_op;
>   
>       /* Initialize DisasContext */
>       db->tb = tb;
> @@ -82,6 +83,7 @@ void translator_loop(const TranslatorOps *ops, 
> DisasContextBase *db,
>   
>       while (true) {
>           db->num_insns++;
> +        last_op = tcg_last_op();
>           ops->insn_start(db, cpu);
>           tcg_debug_assert(db->is_jmp == DISAS_NEXT);  /* no early exit */
>   
> @@ -103,6 +105,12 @@ void translator_loop(const TranslatorOps *ops, DisasContextBase *db,
>               ops->translate_insn(db, cpu);
>           }
>   
> +        if (db->is_jmp == DISAS_TOO_MANY_UNDO) {
> +            db->num_insns--;
> +            tcg_remove_ops_after(last_op);
> +            db->is_jmp = DISAS_TOO_MANY;
> +        }
> +
>           /* Stop translation if translate_insn so indicated.  */
>           if (db->is_jmp != DISAS_NEXT) {
>               break;
> diff --git a/include/exec/translator.h b/include/exec/translator.h
> index d27f8c33b6..e1533aee87 100644
> --- a/include/exec/translator.h
> +++ b/include/exec/translator.h
> @@ -31,6 +31,8 @@
>    * DisasJumpType:
>    * @DISAS_NEXT: Next instruction in program order.
>    * @DISAS_TOO_MANY: Too many instructions translated.
> + * @DISAS_TOO_MANY_UNDO: Too many instructions translated. Everything that was
> + *                       done for the current instruction must be undone.
>    * @DISAS_NORETURN: Following code is dead.
>    * @DISAS_TARGET_*: Start of target-specific conditions.
>    *
> @@ -39,6 +41,7 @@
>   typedef enum DisasJumpType {
>       DISAS_NEXT,
>       DISAS_TOO_MANY,
> +    DISAS_TOO_MANY_UNDO,

Hmm, maybe.  I'm not overly keen on the generic change, because I think it would be easy 
to use incorrectly.

> +    case 2:
> +        /* Restore state that may affect the next instruction. */
> +        s->cc_op_dirty = orig_cc_op_dirty;
> +        s->cc_op = orig_cc_op;
> +        s->base.is_jmp = DISAS_TOO_MANY_UNDO;

I think you can simply set s->prev_insn_end in i386_tr_insn_start, for 
discarding opcodes.


r~