Pavel Dovgalyuk <pavel.dovgal...@ispras.ru> writes:
> When debugging with the watchpoints, qemu may need to create > TB with single instruction. This is achieved by setting cpu->cflags_next_tb. > But when this block is about to execute, it may be interrupted by another > thread. In this case cflags will be lost and next executed TB will not > be the special one. > This patch checks TB exit reason and restores cflags_next_tb to allow > finding the interrupted block. How about this alternative? --8<---------------cut here---------------start------------->8--- accel/tcg: suppress IRQ check for special TBs Generally when we set cpu->cflags_next_tb it is because we want to carefully control the execution of the next TB. Currently there is a race that causes cflags_next_tb to get ignored if an IRQ is processed before we execute any actual instructions. To avoid this we introduce a new compiler flag: CF_NOIRQ to suppress this check in the generated code so we know we will definitely execute the next block. Signed-off-by: Alex Bennée <alex.ben...@linaro.org> Cc: Pavel Dovgalyuk <pavel.dovgal...@ispras.ru> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/245 3 files changed, 22 insertions(+), 3 deletions(-) include/exec/exec-all.h | 1 + include/exec/gen-icount.h | 19 ++++++++++++++++--- accel/tcg/cpu-exec.c | 5 +++++ modified include/exec/exec-all.h @@ -503,6 +503,7 @@ struct TranslationBlock { #define CF_USE_ICOUNT 0x00020000 #define CF_INVALID 0x00040000 /* TB is stale. Set with @jmp_lock held */ #define CF_PARALLEL 0x00080000 /* Generate code for a parallel context */ +#define CF_NOIRQ 0x00100000 /* Generate an uninterruptible TB */ #define CF_CLUSTER_MASK 0xff000000 /* Top 8 bits are cluster ID */ #define CF_CLUSTER_SHIFT 24 modified include/exec/gen-icount.h @@ -21,7 +21,6 @@ static inline void gen_tb_start(const TranslationBlock *tb) { TCGv_i32 count; - tcg_ctx->exitreq_label = gen_new_label(); if (tb_cflags(tb) & CF_USE_ICOUNT) { count = tcg_temp_local_new_i32(); } else { @@ -42,7 +41,19 @@ static inline void gen_tb_start(const TranslationBlock *tb) icount_start_insn = tcg_last_op(); } - tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, tcg_ctx->exitreq_label); + /* + * Emit the check against icount_decr.u32 to see if we should exit + * unless we suppress the check with CF_NOIRQ. If we are using + * icount and have suppressed interruption the higher level code + * should have ensured we don't run more instructions than the + * budget. + */ + if (tb_cflags(tb) & CF_NOIRQ) { + tcg_ctx->exitreq_label = NULL; + } else { + tcg_ctx->exitreq_label = gen_new_label(); + tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, tcg_ctx->exitreq_label); + } if (tb_cflags(tb) & CF_USE_ICOUNT) { tcg_gen_st16_i32(count, cpu_env, @@ -74,7 +85,9 @@ static inline void gen_tb_end(const TranslationBlock *tb, int num_insns) tcgv_i32_arg(tcg_constant_i32(num_insns))); } - gen_set_label(tcg_ctx->exitreq_label); + if (tcg_ctx->exitreq_label) { + gen_set_label(tcg_ctx->exitreq_label); + } tcg_gen_exit_tb(tb, TB_EXIT_REQUESTED); } modified accel/tcg/cpu-exec.c @@ -954,11 +954,16 @@ int cpu_exec(CPUState *cpu) * after-access watchpoints. Since this request should never * have CF_INVALID set, -1 is a convenient invalid value that * does not require tcg headers for cpu_common_reset. + * + * As we don't want this special TB being interrupted by + * some sort of asynchronous event we apply CF_NOIRQ to + * disable the usual event checking. */ cflags = cpu->cflags_next_tb; if (cflags == -1) { cflags = curr_cflags(cpu); } else { + cflags |= CF_NOIRQ; cpu->cflags_next_tb = -1; } --8<---------------cut here---------------end--------------->8--- > > Signed-off-by: Pavel Dovgalyuk <pavel.dovgal...@ispras.ru> > --- > accel/tcg/cpu-exec.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c > index 2d14d02f6c..df12452b8f 100644 > --- a/accel/tcg/cpu-exec.c > +++ b/accel/tcg/cpu-exec.c > @@ -846,6 +846,16 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, > TranslationBlock *tb, > * cpu_handle_interrupt. cpu_handle_interrupt will also > * clear cpu->icount_decr.u16.high. > */ > + if (cpu->cflags_next_tb == -1 > + && (!use_icount || !(tb->cflags & CF_USE_ICOUNT) > + || cpu_neg(cpu)->icount_decr.u16.low >= tb->icount)) { > + /* > + * icount is disabled or there are enough instructions > + * in the budget, do not retranslate this block with > + * different parameters. > + */ > + cpu->cflags_next_tb = tb->cflags; > + } > return; > } > -- Alex Bennée
