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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 25, 2018 at 11:40:12AM +0200, Igor Mammedov wrote:
> On Fri, 22 Jun 2018 22:22:05 +0300
> "Michael S. Tsirkin" <mst@redhat.com> 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 <kernellwp@gmail.com> .
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  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?


Well it also only applies with -cpu host right now.
And guest works fine, even if it's somewhat slower.

As there's no interface to discover which configurations
work, I *suspect* it's easier for management if we keep guest
running rather than fail and make it guess.

> >                  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
> > +@item -overcommit mem-lock=on|off
> > +@item -overcommit cpu-pm=on|off
> > +@findex -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