Re: [Qemu-devel] [PATCH] mttcg/i386: Patch instruction using async_safe_* framework

["Alex Bennée" <alex.bennee@linaro.org>]

Pranith Kumar <bobby.prani@gmail.com> writes:

> In mttcg, calling pause_all_vcpus() during execution from the
> generated TBs causes a deadlock if some vCPU is waiting for exclusive
> execution in start_exclusive(). Fix this by using the aync_safe_*
> framework instead of pausing vcpus for patching instructions.
>
> CC: Richard Henderson <rth@twiddle.net>
> CC: Peter Maydell <peter.maydell@linaro.org>
> CC: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  hw/i386/kvmvapic.c | 104 ++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 76 insertions(+), 28 deletions(-)
>
> diff --git a/hw/i386/kvmvapic.c b/hw/i386/kvmvapic.c
> index 82a4955..15eb39d 100644
> --- a/hw/i386/kvmvapic.c
> +++ b/hw/i386/kvmvapic.c
> @@ -383,8 +383,7 @@ static void patch_byte(X86CPU *cpu, target_ulong addr, uint8_t byte)
>      cpu_memory_rw_debug(CPU(cpu), addr, &byte, 1, 1);
>  }
>
> -static void patch_call(VAPICROMState *s, X86CPU *cpu, target_ulong ip,
> -                       uint32_t target)
> +static void patch_call(X86CPU *cpu, target_ulong ip, uint32_t target)
>  {
>      uint32_t offset;
>
> @@ -393,17 +392,74 @@ static void patch_call(VAPICROMState *s, X86CPU *cpu, target_ulong ip,
>      cpu_memory_rw_debug(CPU(cpu), ip + 1, (void *)&offset, sizeof(offset), 1);
>  }
>
> -static void patch_instruction(VAPICROMState *s, X86CPU *cpu, target_ulong ip)
> +struct PatchInfo {
> +    VAPICHandlers *handler;
> +    target_ulong ip;
> +};
> +
> +static void do_patch_instruction(CPUState *cs, run_on_cpu_data data)
>  {
> -    CPUState *cs = CPU(cpu);
> -    CPUX86State *env = &cpu->env;
> -    VAPICHandlers *handlers;
> +    X86CPU *x86_cpu = X86_CPU(cs);
> +    CPUX86State *env = &x86_cpu->env;
> +    struct PatchInfo *info = (struct PatchInfo *) data.host_ptr;
> +    VAPICHandlers *handlers = info->handler;
> +    target_ulong ip = info->ip;
>      uint8_t opcode[2];
>      uint32_t imm32 = 0;
>      target_ulong current_pc = 0;
>      target_ulong current_cs_base = 0;
>      uint32_t current_flags = 0;
>
> +    cpu_get_tb_cpu_state(env, &current_pc, &current_cs_base,
> +                         &current_flags);
> +    /* Account this instruction, because we will exit the tb.
> +       This is the first instruction in the block. Therefore
> +       there is no need in restoring CPU state. */
> +    if (use_icount) {
> +        --cs->icount_decr.u16.low;
> +    }
> +
> +    cpu_memory_rw_debug(cs, ip, opcode, sizeof(opcode), 0);
> +
> +    switch (opcode[0]) {
> +    case 0x89: /* mov r32 to r/m32 */
> +        patch_byte(x86_cpu, ip, 0x50 + modrm_reg(opcode[1]));  /* push reg */
> +        patch_call(x86_cpu, ip + 1, handlers->set_tpr);
> +        break;
> +    case 0x8b: /* mov r/m32 to r32 */
> +        patch_byte(x86_cpu, ip, 0x90);
> +        patch_call(x86_cpu, ip + 1, handlers->get_tpr[modrm_reg(opcode[1])]);
> +        break;
> +    case 0xa1: /* mov abs to eax */
> +        patch_call(x86_cpu, ip, handlers->get_tpr[0]);
> +        break;
> +    case 0xa3: /* mov eax to abs */
> +        patch_call(x86_cpu, ip, handlers->set_tpr_eax);
> +        break;
> +    case 0xc7: /* mov imm32, r/m32 (c7/0) */
> +        patch_byte(x86_cpu, ip, 0x68);  /* push imm32 */
> +        cpu_memory_rw_debug(cs, ip + 6, (void *)&imm32, sizeof(imm32), 0);
> +        cpu_memory_rw_debug(cs, ip + 1, (void *)&imm32, sizeof(imm32), 1);
> +        patch_call(x86_cpu, ip + 5, handlers->set_tpr);
> +        break;
> +    case 0xff: /* push r/m32 */
> +        patch_byte(x86_cpu, ip, 0x50); /* push eax */
> +        patch_call(x86_cpu, ip + 1, handlers->get_tpr_stack);
> +        break;
> +    default:
> +        abort();
> +    }
> +
> +    g_free(info);
> +}
> +
> +static void patch_instruction(VAPICROMState *s, X86CPU *cpu, target_ulong ip)
> +{
> +    CPUState *cs = CPU(cpu);
> +    VAPICHandlers *handlers;
> +    uint8_t opcode[2];
> +    uint32_t imm32 = 0;
> +
>      if (smp_cpus == 1) {
>          handlers = &s->rom_state.up;
>      } else {
> @@ -411,14 +467,14 @@ static void patch_instruction(VAPICROMState *s, X86CPU *cpu, target_ulong ip)
>      }
>
>      if (!kvm_enabled()) {
> -        cpu_get_tb_cpu_state(env, &current_pc, &current_cs_base,
> -                             &current_flags);
> -        /* Account this instruction, because we will exit the tb.
> -           This is the first instruction in the block. Therefore
> -           there is no need in restoring CPU state. */
> -        if (use_icount) {
> -            --cs->icount_decr.u16.low;
> -        }
> +        struct PatchInfo *info = g_new(struct PatchInfo, 1);
> +        const run_on_cpu_func fn = do_patch_instruction;
> +        info->handler = handlers;
> +        info->ip = ip;
> +
> +        async_safe_run_on_cpu(cs, fn, RUN_ON_CPU_HOST_PTR(info));
> +        cs->exception_index = EXCP_INTERRUPT;
> +        cpu_loop_exit(cs);

So my only concern is has the path we've taken here ensured that all
registers have been updated before we leave the loop. That said we
previously called cpu_loop_exit_noexc() which has the lovely comment:

  void cpu_loop_exit_noexc(CPUState *cpu)
  {
      /* XXX: restore cpu registers saved in host registers */

But the offloading of this work to async is the right approach.

>      }
>
>      pause_all_vcpus();
> @@ -428,41 +484,33 @@ static void patch_instruction(VAPICROMState *s, X86CPU *cpu, target_ulong ip)
>      switch (opcode[0]) {
>      case 0x89: /* mov r32 to r/m32 */
>          patch_byte(cpu, ip, 0x50 + modrm_reg(opcode[1]));  /* push reg */
> -        patch_call(s, cpu, ip + 1, handlers->set_tpr);
> +        patch_call(cpu, ip + 1, handlers->set_tpr);
>          break;
>      case 0x8b: /* mov r/m32 to r32 */
>          patch_byte(cpu, ip, 0x90);
> -        patch_call(s, cpu, ip + 1, handlers->get_tpr[modrm_reg(opcode[1])]);
> +        patch_call(cpu, ip + 1, handlers->get_tpr[modrm_reg(opcode[1])]);
>          break;
>      case 0xa1: /* mov abs to eax */
> -        patch_call(s, cpu, ip, handlers->get_tpr[0]);
> +        patch_call(cpu, ip, handlers->get_tpr[0]);
>          break;
>      case 0xa3: /* mov eax to abs */
> -        patch_call(s, cpu, ip, handlers->set_tpr_eax);
> +        patch_call(cpu, ip, handlers->set_tpr_eax);
>          break;
>      case 0xc7: /* mov imm32, r/m32 (c7/0) */
>          patch_byte(cpu, ip, 0x68);  /* push imm32 */
>          cpu_memory_rw_debug(cs, ip + 6, (void *)&imm32, sizeof(imm32), 0);
>          cpu_memory_rw_debug(cs, ip + 1, (void *)&imm32, sizeof(imm32), 1);
> -        patch_call(s, cpu, ip + 5, handlers->set_tpr);
> +        patch_call(cpu, ip + 5, handlers->set_tpr);
>          break;
>      case 0xff: /* push r/m32 */
>          patch_byte(cpu, ip, 0x50); /* push eax */
> -        patch_call(s, cpu, ip + 1, handlers->get_tpr_stack);
> +        patch_call(cpu, ip + 1, handlers->get_tpr_stack);
>          break;
>      default:
>          abort();
>      }
>
>      resume_all_vcpus();
> -
> -    if (!kvm_enabled()) {
> -        /* Both tb_lock and iothread_mutex will be reset when
> -         *  longjmps back into the cpu_exec loop. */
> -        tb_lock();
> -        tb_gen_code(cs, current_pc, current_cs_base, current_flags, 1);
> -        cpu_loop_exit_noexc(cs);
> -    }
>  }
>
>  void vapic_report_tpr_access(DeviceState *dev, CPUState *cs, target_ulong ip,


--
Alex Bennée