Re: [Qemu-devel] [PATCH v6 1/2] kvm: support -overcommit cpu-pm=on|off

[Igor Mammedov]

On Fri, 22 Jun 2018 22:22:05 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> With this flag, kvm allows guest to control host CPU power state.  This
> increases latency for other processes using same host CPU in an
> unpredictable way, but if decreases idle entry/exit times for the
> running VCPU, so to use it QEMU needs a hint about whether host CPU is
> overcommitted, hence the flag name.
> 
> Follow-up patches will expose this capability to guest
> (using mwait leaf).
> 
> Based on a patch by Wanpeng Li <address@hidden> .
> 
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
>  include/sysemu/sysemu.h |  1 +
>  target/i386/kvm.c       | 23 +++++++++++++++++++++++
>  vl.c                    | 32 +++++++++++++++++++++++++++++++-
>  qemu-options.hx         | 27 +++++++++++++++++++++++++--
>  4 files changed, 80 insertions(+), 3 deletions(-)
> 
> diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> index e893f72f3b..b921c6f3b7 100644
> --- a/include/sysemu/sysemu.h
> +++ b/include/sysemu/sysemu.h
> @@ -128,6 +128,7 @@ extern bool boot_strict;
>  extern uint8_t *boot_splash_filedata;
>  extern size_t boot_splash_filedata_size;
>  extern bool enable_mlock;
> +extern bool enable_cpu_pm;
>  extern uint8_t qemu_extra_params_fw[2];
>  extern QEMUClockType rtc_clock;
>  extern const char *mem_path;
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index 44f70733e7..cf9107be4b 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -1357,6 +1357,29 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>          smram_machine_done.notify = register_smram_listener;
>          qemu_add_machine_init_done_notifier(&smram_machine_done);
>      }
> +
> +    if (enable_cpu_pm) {
> +        int disable_exits = kvm_check_extension(s, 
> KVM_CAP_X86_DISABLE_EXITS);
> +        int ret;
> +
> +/* Work around for kernel header with a typo. TODO: fix header and drop. */
> +#if defined(KVM_X86_DISABLE_EXITS_HTL) && !defined(KVM_X86_DISABLE_EXITS_HLT)
> +#define KVM_X86_DISABLE_EXITS_HLT KVM_X86_DISABLE_EXITS_HTL
> +#endif
> +        if (disable_exits) {
> +            disable_exits &= (KVM_X86_DISABLE_EXITS_MWAIT |
> +                              KVM_X86_DISABLE_EXITS_HLT |
> +                              KVM_X86_DISABLE_EXITS_PAUSE);
> +        }
> +
> +        ret = kvm_vm_enable_cap(s, KVM_CAP_X86_DISABLE_EXITS, 0,
> +                                disable_exits);
> +        if (ret < 0) {
> +            error_report("kvm: guest stopping CPU not supported: %s",
> +                         strerror(-ret));
> +        }
> +    }
> +
>      return 0;
>  }
>  
> diff --git a/vl.c b/vl.c
> index 06031715ac..c9530efed5 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -142,6 +142,7 @@ ram_addr_t ram_size;
>  const char *mem_path = NULL;
>  int mem_prealloc = 0; /* force preallocation of physical target memory */
>  bool enable_mlock = false;
> +bool enable_cpu_pm = false;
>  int nb_nics;
>  NICInfo nd_table[MAX_NICS];
>  int autostart;
> @@ -390,6 +391,22 @@ static QemuOptsList qemu_realtime_opts = {
>      },
>  };
>  
> +static QemuOptsList qemu_overcommit_opts = {
> +    .name = "overcommit",
> +    .head = QTAILQ_HEAD_INITIALIZER(qemu_overcommit_opts.head),
> +    .desc = {
> +        {
> +            .name = "mem-lock",
> +            .type = QEMU_OPT_BOOL,
> +        },
> +        {
> +            .name = "cpu-pm",
> +            .type = QEMU_OPT_BOOL,
> +        },
> +        { /* end of list */ }
> +    },
> +};
> +
>  static QemuOptsList qemu_msg_opts = {
>      .name = "msg",
>      .head = QTAILQ_HEAD_INITIALIZER(qemu_msg_opts.head),
> @@ -3903,7 +3920,20 @@ int main(int argc, char **argv, char **envp)
>                  if (!opts) {
>                      exit(1);
>                  }
> -                enable_mlock = qemu_opt_get_bool(opts, "mlock", true);
> +                /* Don't override the -overcommit option if set */
> +                enable_mlock = enable_mlock ||
> +                    qemu_opt_get_bool(opts, "mlock", true);
> +                break;
> +            case QEMU_OPTION_overcommit:
> +                opts = qemu_opts_parse_noisily(qemu_find_opts("overcommit"),
> +                                               optarg, false);
> +                if (!opts) {
> +                    exit(1);
> +                }
> +                /* Don't override the -realtime option if set */
> +                enable_mlock = enable_mlock ||
> +                    qemu_opt_get_bool(opts, "mem-lock", false);
> +                enable_cpu_pm = qemu_opt_get_bool(opts, "cpu-pm", false);
should we error out or complain if the option can't be used
/i.e. in case of non kvm accelerator/
instead of silently ignoring it and making user wonder why it doesn't work?

>                  break;
>              case QEMU_OPTION_msg:
>                  opts = qemu_opts_parse_noisily(qemu_find_opts("msg"), optarg,
> diff --git a/qemu-options.hx b/qemu-options.hx
> index c0d3951e9f..1bba3d258b 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3328,8 +3328,7 @@ DEF("realtime", HAS_ARG, QEMU_OPTION_realtime,
>      "-realtime [mlock=on|off]\n"
>      "                run qemu with realtime features\n"
>      "                mlock=on|off controls mlock support (default: on)\n",
> -    QEMU_ARCH_ALL)
> -STEXI
> +    QEMU_ARCH_ALL) STEXI
>  @item -realtime mlock=on|off
>  @findex -realtime
>  Run qemu with realtime features.
> @@ -3337,6 +3336,30 @@ mlocking qemu and guest memory can be enabled via 
> @option{mlock=on}
>  (enabled by default).
>  ETEXI
>  
> +DEF("overcommit", HAS_ARG, QEMU_OPTION_overcommit,
> +    "--overcommit [mem-lock=on|off][cpu-pm=on|off]\n"
> +    "                run qemu with overcommit hints\n"
> +    "                mem-lock=on|off controls memory lock support (default: 
> off)\n"
> +    "                cpu-pm=on|off controls cpu power management (default: 
> off)\n",
> +    QEMU_ARCH_ALL)
> +STEXI
> address@hidden -overcommit mem-lock=on|off
> address@hidden -overcommit cpu-pm=on|off
> address@hidden -overcommit
> +Run qemu with hints about host resource overcommit. The default is
> +to assume that host overcommits all resources.
> +
> +Locking qemu and guest memory can be enabled via @option{mem-lock=on} 
> (disabled
> +by default).  This works when host memory is not overcommitted and reduces 
> the
> +worst-case latency for guest.  This is equivalent to @option{realtime}.
> +
> +Guest ability to manage power state of host cpus (increasing latency for 
> other
> +processes on the same host cpu, but decreasing latency for guest) can be
> +enabled via @option{cpu-pm=on} (disabled by default).  This works best when
> +host CPU is not overcommitted. When used, host estimates of CPU cycle and 
> power
> +utilization will be incorrect, not taking into account guest idle time.
> +ETEXI
> +
>  DEF("gdb", HAS_ARG, QEMU_OPTION_gdb, \
>      "-gdb dev        wait for gdb connection on 'dev'\n", QEMU_ARCH_ALL)
>  STEXI