Re: [PATCH v3 2/4] hw/acpi: Introduce the QEMU Battery

["Denis V. Lunev" <dlunev@gmail.com>]

On 8/28/25 00:00, Leonid Bloch wrote:
> The battery device communicates battery state to the guest via ACPI.
> It supports two modes of operation:
>
> 1. QMP control mode (default): Battery state is controlled programmatically
>     via QMP commands, making the device deterministic and migration-safe.
>
> 2. Host mirroring mode (optional): The device reflects the host's battery
>     state from sysfs. Probing occurs on guest ACPI requests and at timed
>     intervals (default 2 seconds, configurable via 'probe_interval' property
>     in milliseconds). State changes trigger ACPI notifications to the guest.
>
> Properties:
> - 'use-qmp': Enable QMP control mode (default: true)
> - 'enable-sysfs': Enable host battery mirroring (default: false)
> - 'probe_interval': Probe interval in ms for sysfs mode (default: 2000)
> - 'sysfs_path': Override default sysfs path /sys/class/power_supply/
>
> The device implements the ACPI_DEV_AML_IF interface to generate its
> own AML code, placing the BAT0 device directly under \_SB scope as
> per ACPI specification.
>
> QMP commands:
> - battery-set-state: Set battery state (present, charging, capacity, rate)
> - query-battery: Query current battery state
>
> This allows testing without host battery access and provides a stable
> interface for virtualization management systems.
>
> Signed-off-by: Leonid Bloch <lb.workbox@gmail.com>
> Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> ---
>   MAINTAINERS                          |   6 +
>   docs/specs/battery.rst               | 225 ++++++++
>   docs/specs/index.rst                 |   1 +
>   hw/acpi/Kconfig                      |   4 +
>   hw/acpi/battery.c                    | 735 +++++++++++++++++++++++++++
>   hw/acpi/meson.build                  |   1 +
>   hw/acpi/trace-events                 |   5 +
>   hw/i386/Kconfig                      |   1 +
>   hw/i386/acpi-build.c                 |   1 +
>   include/hw/acpi/acpi_dev_interface.h |   1 +
>   include/hw/acpi/battery.h            |  33 ++
>   qapi/acpi.json                       |  73 +++
>   12 files changed, 1086 insertions(+)
>   create mode 100644 docs/specs/battery.rst
>   create mode 100644 hw/acpi/battery.c
>   create mode 100644 include/hw/acpi/battery.h
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index a64b5b849b..eb71a4a4b7 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -2928,6 +2928,12 @@ F: hw/vmapple/*
>   F: include/hw/vmapple/*
>   F: docs/system/arm/vmapple.rst
>   
> +Battery
> +M: Leonid Bloch <lb.workbox@gmail.com>
> +S: Maintained
> +F: hw/acpi/battery.*
> +F: docs/specs/battery.rst
> +
>   Subsystems
>   ----------
>   Overall Audio backends
> diff --git a/docs/specs/battery.rst b/docs/specs/battery.rst
> new file mode 100644
> index 0000000000..e426b91ea5
> --- /dev/null
> +++ b/docs/specs/battery.rst
> @@ -0,0 +1,225 @@
> +.. SPDX-License-Identifier: GPL-2.0-or-later
> +
> +==============
> +Battery Device
> +==============
> +
> +The battery device provides battery state information to the guest. It supports
> +two operating modes:
> +
> +1. **QMP Control Mode** (default): Battery state is controlled via QMP commands,
> +   providing deterministic control for testing and migration safety.
> +2. **Sysfs Mode**: Battery state mirrors the host's physical battery, useful
> +   for desktop virtualization where the guest should see the host's battery.
> +
> +Configuration
> +-------------
> +
> +The battery device is created as an ISA device using ``-device battery``.
> +
> +Operating Modes
> +~~~~~~~~~~~~~~~
> +
> +**QMP Control Mode** (``use-qmp=true``, default)
> +  Battery state is controlled via QMP commands. This mode is recommended for:
> +
> +  * Production environments requiring migration support
> +  * Testing with predictable battery states
> +  * Environments without host battery access
> +  * Security-sensitive deployments
> +
> +**Sysfs Mode** (``enable-sysfs=true``)
> +  Battery mirrors the host's physical battery. This mode is useful for:
> +
> +  * Desktop virtualization on laptops
> +  * Development and testing with real battery behavior
> +
> +  Note: Sysfs mode reads host files and runs timers, which may impact
> +  security and migration. Use with caution in production.
> +
> +Properties
> +~~~~~~~~~~
> +
> +``ioport`` (default: 0x530)
> +  I/O port base address for the battery device registers.
> +
> +``use-qmp`` (default: true)
> +  Enable QMP control mode. When true, battery state is controlled via
> +  QMP commands. Cannot be used together with ``enable-sysfs=true``.
> +
> +``enable-sysfs`` (default: false)
> +  Enable sysfs mode to mirror the host's battery. Cannot be used together
> +  with ``use-qmp=true``.
> +
> +``probe_interval`` (default: 2000)
> +  Time interval between periodic probes in milliseconds (sysfs mode only).
> +  A zero value disables the periodic probes, and makes the battery state
> +  updates occur on guest requests only.
> +
> +``sysfs_path`` (default: auto-detected)
> +  Path to the host's battery sysfs directory (sysfs mode only). If not
> +  specified, the device will automatically detect the battery from
> +  ``/sys/class/power_supply/``. This property allows overriding the default
> +  path if:
> +
> +  * The sysfs path differs from the standard location
> +  * A different battery needs to be probed
> +  * A "fake" host battery is to be provided for testing
> +
> +Host Battery Detection
> +----------------------
> +
> +The host's battery information is taken from the sysfs battery data,
> +located in::
> +
> +  /sys/class/power_supply/[device of type "Battery"]
> +
> +The device automatically scans for the first available battery device
> +with type "Battery" in the power_supply directory.
> +
> +ACPI Interface
> +--------------
> +
> +The battery device exposes itself as an ACPI battery device with:
> +
> +* **_HID**: ``PNP0C0A`` (Control Method Battery)
> +* **Device path**: ``\_SB.BAT0``
> +
> +The device implements standard ACPI battery methods:
> +
> +``_STA`` (Status)
> +  Returns the battery presence status.
> +
> +``_BIF`` (Battery Information)
> +  Returns static battery information including design capacity,
> +  technology, and model information.
> +
> +``_BST`` (Battery Status)
> +  Returns dynamic battery status including current state
> +  (charging/discharging), present rate, and remaining capacity.
> +
> +I/O Interface
> +-------------
> +
> +The battery device exposes 12 bytes of I/O space at the configured
> +I/O port address with the following layout:
> +
> +* **Bytes 0-3**: Battery state (DWORD)
> +
> +  * 1 = Discharging
> +  * 2 = Charging
> +
> +* **Bytes 4-7**: Battery rate (DWORD)
> +
> +  Current charge/discharge rate normalized to design capacity.
> +
> +* **Bytes 8-11**: Battery charge (DWORD)
> +
> +  Current battery charge level normalized to design capacity.
> +
> +All values are normalized where the design capacity equals 10000 units.
> +Unknown values are represented as 0xFFFFFFFF.
> +
> +ACPI Notifications
> +------------------
> +
> +The battery device generates ACPI notifications through GPE events:
> +
> +* **GPE._E07**: Device Check (0x01) - Battery insertion/removal
> +* **GPE._E08**: Status Change (0x80) - Battery state change
> +* **GPE._E09**: Information Change (0x81) - Battery information update
> +
> +QMP Commands
> +------------
> +
> +When using QMP control mode (default), the following commands are available:
> +
> +``battery-set-state``
> +  Set the battery state. Parameters:
> +
> +  * ``present``: Whether the battery is present (boolean)
> +  * ``charging``: Whether the battery is charging (boolean)
> +  * ``discharging``: Whether the battery is discharging (boolean)
> +  * ``charge-percent``: Battery charge percentage 0-100 (integer)
> +  * ``rate``: Charge/discharge rate in mW (optional integer)
> +
> +  Example::
> +
> +    -> { "execute": "battery-set-state",
> +         "arguments": { "state": {
> +           "present": true,
> +           "charging": true,
> +           "discharging": false,
> +           "charge-percent": 85,
> +           "rate": 500
> +         }}}
> +    <- { "return": {} }
> +
> +``query-battery``
> +  Query the current battery state. Returns the same fields as above.
> +
> +  Example::
> +
> +    -> { "execute": "query-battery" }
> +    <- { "return": {
> +           "present": true,
> +           "charging": true,
> +           "discharging": false,
> +           "charge-percent": 85,
> +           "rate": 500,
> +           "design-capacity": 10000
> +         }}
> +
> +Examples
> +--------
> +
> +QMP control mode (default - recommended)::
> +
> +  # Start with QMP control
> +  qemu-system-x86_64 -device battery -qmp tcp:localhost:4444,server,wait=off
> +
> +  # From another terminal, set battery state via QMP:
> +  echo '{"execute":"qmp_capabilities"}
> +        {"execute":"battery-set-state",
> +         "arguments":{"state":{"present":true,"charging":false,
> +                               "discharging":true,"charge-percent":42,
> +                               "rate":500}}}' | \
> +  nc -N localhost 4444
> +
> +Sysfs mode (mirror host battery)::
> +
> +  # Enable sysfs mode to mirror host battery
> +  qemu-system-x86_64 -device battery,use-qmp=false,enable-sysfs=true
> +
> +  # Custom probe interval (5 seconds)
> +  qemu-system-x86_64 -device battery,use-qmp=false,enable-sysfs=true,probe_interval=5000
> +
> +  # Specific battery path
> +  qemu-system-x86_64 -device battery,use-qmp=false,enable-sysfs=true,sysfs_path=/sys/class/power_supply/BAT1
> +
> +Testing with fake battery::
> +
> +  # Create fake battery files for testing
> +  mkdir -p /tmp/fake_battery
> +  echo "Battery" > /tmp/fake_battery/type
> +  echo "Discharging" > /tmp/fake_battery/status
> +  echo "50" > /tmp/fake_battery/capacity
> +  echo "1500000" > /tmp/fake_battery/energy_now    # Current energy in μWh
> +  echo "3000000" > /tmp/fake_battery/energy_full   # Full capacity in μWh
> +  echo "500000" > /tmp/fake_battery/power_now      # Current power in μW
> +
> +  # Use fake battery in sysfs mode
> +  qemu-system-x86_64 -device battery,use-qmp=false,enable-sysfs=true,sysfs_path=/tmp/fake_battery
> +
> +  # Update battery state while VM is running (from another terminal)
> +  # Change to 75% charging:
> +  echo "Charging" > /tmp/fake_battery/status
> +  echo "75" > /tmp/fake_battery/capacity
> +  echo "2250000" > /tmp/fake_battery/energy_now    # 75% of 3000000
> +  echo "500000" > /tmp/fake_battery/power_now      # Charging rate (500 mW)
> +
> +  # Change to 25% discharging:
> +  echo "Discharging" > /tmp/fake_battery/status
> +  echo "25" > /tmp/fake_battery/capacity
> +  echo "750000" > /tmp/fake_battery/energy_now     # 25% of 3000000
> +  echo "300000" > /tmp/fake_battery/power_now      # Discharge rate (300 mW)
> diff --git a/docs/specs/index.rst b/docs/specs/index.rst
> index f19d73c9f6..616e8228cc 100644
> --- a/docs/specs/index.rst
> +++ b/docs/specs/index.rst
> @@ -22,6 +22,7 @@ guest hardware that is specific to QEMU.
>      acpi_pci_hotplug
>      acpi_nvdimm
>      acpi_erst
> +   battery
>      sev-guest-firmware
>      fw_cfg
>      fsi
> diff --git a/hw/acpi/Kconfig b/hw/acpi/Kconfig
> index 1d4e9f0845..64403378bd 100644
> --- a/hw/acpi/Kconfig
> +++ b/hw/acpi/Kconfig
> @@ -83,3 +83,7 @@ config ACPI_ERST
>   config ACPI_CXL
>       bool
>       depends on ACPI
> +
> +config BATTERY
> +    bool
> +    depends on ACPI
> diff --git a/hw/acpi/battery.c b/hw/acpi/battery.c
> new file mode 100644
> index 0000000000..5ab3ab7472
> --- /dev/null
> +++ b/hw/acpi/battery.c
> @@ -0,0 +1,735 @@
> +/*
> + * QEMU emulated battery device.
> + *
> + * Copyright (c) 2019 Janus Technologies, Inc. (http://janustech.com)
> + *
> + * Authors:
> + *     Leonid Bloch <lb.workbox@gmail.com>
> + *     Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> + *     Dmitry Fleytman <dmitry.fleytman@gmail.com>
> + *
> + * SPDX-License-Identifier: GPL-2.0-or-later
> + *
> + */
> +
> +#include "qemu/osdep.h"
> +#include "trace.h"
> +#include "hw/isa/isa.h"
> +#include "hw/acpi/acpi.h"
> +#include "hw/nvram/fw_cfg.h"
> +#include "qapi/error.h"
> +#include "qemu/error-report.h"
> +#include "hw/qdev-properties.h"
> +#include "migration/vmstate.h"
> +#include "hw/acpi/acpi_aml_interface.h"
> +#include "qapi/qapi-commands-acpi.h"
> +
> +#include "hw/acpi/battery.h"
> +
> +#define BATTERY_DEVICE(obj) OBJECT_CHECK(BatteryState, (obj), TYPE_BATTERY)
> +
> +#define BATTERY_DISCHARGING  0x01  /* ACPI _BST bit 0 */
> +#define BATTERY_CHARGING     0x02  /* ACPI _BST bit 1 */
> +#define BATTERY_CRITICAL     0x04  /* ACPI _BST bit 2 */
> +
> +#define SYSFS_PATH       "/sys/class/power_supply"
> +#define BATTERY_TYPE     "Battery"
> +
> +#define MAX_ALLOWED_STATE_LENGTH  32  /* For convinience when comparing */
> +
> +#define NORMALIZE_BY_FULL(val, full) \
> +    ((full == 0) ? BATTERY_VAL_UNKNOWN \
> +     : (uint32_t)(val * BATTERY_FULL_CAP / full))
> +
> +typedef union bat_metric {
> +    uint32_t val;
> +    uint8_t acc[4];
> +} bat_metric;
> +
> +typedef struct BatteryState {
> +    ISADevice dev;
> +    MemoryRegion io;
> +    uint16_t ioport;
> +    bat_metric state;
> +    bat_metric rate;
> +    bat_metric charge;
> +    uint32_t charge_full;
> +    int units;  /* 0 - mWh, 1 - mAh */
> +    bool use_qmp_control;
> +    bool qmp_present;
> +    bool qmp_charging;
> +    bool qmp_discharging;
> +    int qmp_charge_percent;
> +    int qmp_rate;
> +    bool enable_sysfs;
> +
> +    QEMUTimer *probe_state_timer;
> +    uint64_t probe_state_interval;
> +
> +    char *bat_path;
> +} BatteryState;
> +
> +/* Access addresses */
> +enum acc_addr {
> +    bsta_addr0, bsta_addr1, bsta_addr2, bsta_addr3,
> +    brte_addr0, brte_addr1, brte_addr2, brte_addr3,
> +    bcrg_addr0, bcrg_addr1, bcrg_addr2, bcrg_addr3
> +};
> +
> +/* Files used when the units are:      mWh             mAh      */
> +static const char *full_file[] = { "energy_full", "charge_full" };
> +static const char *now_file[]  = { "energy_now",  "charge_now"  };
> +static const char *rate_file[] = { "power_now",   "current_now" };
> +
> +static const char *stat_file = "status";
> +static const char *type_file = "type";
> +
> +static const char *discharging_states[] = { "Discharging", "Not charging" };
> +static const char *charging_states[] = { "Charging", "Full", "Unknown" };
> +
> +static inline bool battery_file_accessible(char *path, const char *file)
> +{
> +    char full_path[PATH_MAX];
> +    int path_len;
> +
> +    path_len = snprintf(full_path, PATH_MAX, "%s/%s", path, file);
> +    if (path_len < 0 || path_len >= PATH_MAX) {
> +        return false;
> +    }
> +    if (access(full_path, R_OK) == 0) {
> +        return true;
> +    }
> +    return false;
> +}
> +
> +static inline int battery_select_file(char *path, const char **file)
> +{
> +    if (battery_file_accessible(path, file[0])) {
> +        return 0;
> +    } else if (battery_file_accessible(path, file[1])) {
> +        return 1;
> +    } else {
> +        return -1;
> +    }
> +}
> +
> +static void battery_get_full_charge(BatteryState *s, Error **errp)
> +{
> +    char file_path[PATH_MAX];
> +    int path_len;
> +    uint32_t val;
> +    FILE *ff;
> +
> +    path_len = snprintf(file_path, PATH_MAX, "%s/%s", s->bat_path,
> +                        full_file[s->units]);
> +    if (path_len < 0 || path_len >= PATH_MAX) {
> +        error_setg(errp, "Full capacity file path is inaccessible.");
> +        return;
> +    }
> +
> +    ff = fopen(file_path, "r");
> +    if (ff == NULL) {
> +        error_setg_errno(errp, errno, "Could not read the full charge file.");
> +        return;
> +    }
> +
> +    if (fscanf(ff, "%u", &val) != 1) {
> +        error_setg(errp, "Full capacity undetermined.");
> +        return;
> +    } else {
> +        s->charge_full = val;
> +    }
> +    fclose(ff);
> +}
> +
> +static inline bool battery_is_discharging(char *val)
> +{
> +    static const int discharging_len = ARRAY_SIZE(discharging_states);
> +    int i;
> +
> +    for (i = 0; i < discharging_len; i++) {
> +        if (!strncmp(val, discharging_states[i], MAX_ALLOWED_STATE_LENGTH)) {
> +            return true;
> +        }
> +    }
> +    return false;
> +}
> +
> +static inline bool battery_is_charging(char *val)
> +{
> +    static const int charging_len = ARRAY_SIZE(charging_states);
> +    int i;
> +
> +    for (i = 0; i < charging_len; i++) {
> +        if (!strncmp(val, charging_states[i], MAX_ALLOWED_STATE_LENGTH)) {
> +            return true;
> +        }
> +    }
> +    return false;
> +}
> +
> +static void battery_get_state(BatteryState *s)
> +{
> +    char file_path[PATH_MAX];
> +    int path_len;
> +    char val[MAX_ALLOWED_STATE_LENGTH];
> +    FILE *ff;
> +
> +    path_len = snprintf(file_path, PATH_MAX, "%s/%s", s->bat_path, stat_file);
> +    if (path_len < 0 || path_len >= PATH_MAX) {
> +        warn_report("Could not read the battery state.");
> +        return;
> +    }
> +
> +    ff = fopen(file_path, "r");
> +    if (ff == NULL) {
> +        warn_report("Could not read the battery state.");
> +        return;
> +    }
> +
> +    if (fgets(val, MAX_ALLOWED_STATE_LENGTH, ff) == NULL) {
> +        warn_report("Battery state unreadable.");
> +    } else {
> +        val[strcspn(val, "\n")] = 0;
> +        if (battery_is_discharging(val)) {
> +            s->state.val = BATTERY_DISCHARGING;
> +        } else if (battery_is_charging(val)) {
> +            s->state.val = BATTERY_CHARGING;
> +        } else {
> +            s->state.val = 0;
> +            warn_report("Battery state undetermined.");
> +        }
> +    }
> +    fclose(ff);
> +}
> +
> +static void battery_get_rate(BatteryState *s)
> +{
> +    char file_path[PATH_MAX];
> +    int path_len;
> +    uint64_t val;
> +    FILE *ff;
> +
> +    path_len = snprintf(file_path, PATH_MAX, "%s/%s", s->bat_path,
> +                        rate_file[s->units]);
> +    if (path_len < 0 || path_len >= PATH_MAX) {
> +        warn_report("Could not read the battery rate.");
> +        s->rate.val = BATTERY_VAL_UNKNOWN;
> +        return;
> +    }
> +
> +    ff = fopen(file_path, "r");
> +    if (ff == NULL) {
> +        warn_report("Could not read the battery rate.");
> +        s->rate.val = BATTERY_VAL_UNKNOWN;
> +        return;
> +    }
> +
> +    if (fscanf(ff, "%lu", &val) != 1) {
> +        warn_report("Battery rate undetermined.");
> +        s->rate.val = BATTERY_VAL_UNKNOWN;
> +    } else {
> +        s->rate.val = NORMALIZE_BY_FULL(val, s->charge_full);
> +    }
> +    fclose(ff);
> +}
> +
> +static void battery_get_charge(BatteryState *s)
> +{
> +    char file_path[PATH_MAX];
> +    int path_len;
> +    uint64_t val;
> +    FILE *ff;
> +
> +    path_len = snprintf(file_path, PATH_MAX, "%s/%s", s->bat_path,
> +                        now_file[s->units]);
> +    if (path_len < 0 || path_len >= PATH_MAX) {
> +        warn_report("Could not read the battery charge.");
> +        s->charge.val = BATTERY_VAL_UNKNOWN;
> +        return;
> +    }
> +
> +    ff = fopen(file_path, "r");
> +    if (ff == NULL) {
> +        warn_report("Could not read the battery charge.");
> +        s->charge.val = BATTERY_VAL_UNKNOWN;
> +        return;
> +    }
> +
> +    if (fscanf(ff, "%lu", &val) != 1) {
> +        warn_report("Battery charge undetermined.");
> +        s->charge.val = BATTERY_VAL_UNKNOWN;
> +    } else {
> +        s->charge.val = NORMALIZE_BY_FULL(val, s->charge_full);
> +    }
> +    fclose(ff);
> +}
> +
> +static void battery_get_dynamic_status(BatteryState *s)
> +{
> +    if (s->use_qmp_control) {
> +        s->state.val = 0;
> +        if (s->qmp_present) {
> +            if (s->qmp_charging) {
> +                s->state.val |= BATTERY_CHARGING;
> +            }
> +            if (s->qmp_discharging) {
> +                s->state.val |= BATTERY_DISCHARGING;
> +            }
> +        }
> +        s->rate.val = s->qmp_rate;
> +        s->charge.val = (s->qmp_charge_percent * BATTERY_FULL_CAP) / 100;
> +    } else if (s->enable_sysfs) {
> +        battery_get_state(s);
> +        battery_get_rate(s);
> +        battery_get_charge(s);
> +    } else {
> +        s->state.val = 0;
> +        s->rate.val = 0;
> +        s->charge.val = 0;
> +    }
> +
> +    trace_battery_get_dynamic_status(s->state.val, s->rate.val, s->charge.val);
> +}
> +
> +static void battery_probe_state(void *opaque)
> +{
> +    BatteryState *s = opaque;
> +
> +    uint32_t state_before = s->state.val;
> +    uint32_t rate_before = s->rate.val;
> +    uint32_t charge_before = s->charge.val;
> +
> +    battery_get_dynamic_status(s);
> +
> +    if (state_before != s->state.val || rate_before != s->rate.val ||
> +        charge_before != s->charge.val) {
> +        Object *obj = object_resolve_path_type("", TYPE_ACPI_DEVICE_IF, NULL);
> +        switch (charge_before) {
> +        case 0:
> +            break;  /* Avoid marking initiation as an update */
> +        default:
> +            acpi_send_event(DEVICE(obj), ACPI_BATTERY_CHANGE_STATUS);
> +        }
> +    }
> +    timer_mod(s->probe_state_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
> +              s->probe_state_interval);
> +}
> +
> +static void battery_probe_state_timer_init(BatteryState *s)
> +{
> +    if (s->enable_sysfs && s->probe_state_interval > 0) {
> +        s->probe_state_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL,
> +                                            battery_probe_state, s);
> +        timer_mod(s->probe_state_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
> +                  s->probe_state_interval);
> +    }
> +}
> +
> +static bool battery_verify_sysfs(BatteryState *s, char *path)
> +{
> +    int units;
> +    FILE *ff;
> +    char type_path[PATH_MAX];
> +    int path_len;
> +    char val[MAX_ALLOWED_STATE_LENGTH];
> +
> +    path_len = snprintf(type_path, PATH_MAX, "%s/%s", path, type_file);
> +    if (path_len < 0 || path_len >= PATH_MAX) {
> +        return false;
> +    }
> +    ff = fopen(type_path, "r");
> +    if (ff == NULL) {
> +        return false;
> +    }
> +
> +    if (fgets(val, MAX_ALLOWED_STATE_LENGTH, ff) == NULL) {
> +        fclose(ff);
> +        return false;
> +    } else {
> +        val[strcspn(val, "\n")] = 0;
> +        if (strncmp(val, BATTERY_TYPE, MAX_ALLOWED_STATE_LENGTH)) {
> +            fclose(ff);
> +            return false;
> +        }
> +    }
> +    fclose(ff);
> +
> +    units = battery_select_file(path, full_file);
> +
> +    if (units < 0) {
> +        return false;
> +    } else {
> +        s->units = units;
> +    }
> +
> +    return (battery_file_accessible(path, now_file[s->units])
> +            & battery_file_accessible(path, rate_file[s->units])
> +            & battery_file_accessible(path, stat_file));
> +}
> +
> +static bool get_battery_path(DeviceState *dev)
> +{
> +    BatteryState *s = BATTERY_DEVICE(dev);
> +    DIR *dir;
> +    struct dirent *ent;
> +    char bp[PATH_MAX];
> +    int path_len;
> +
> +    if (s->bat_path) {
> +        return battery_verify_sysfs(s, s->bat_path);
> +    }
> +
> +    dir = opendir(SYSFS_PATH);
> +    if (dir == NULL) {
> +        return false;
> +    }
> +
> +    ent = readdir(dir);
> +    while (ent != NULL) {
> +        if (ent->d_name[0] != '.') {
> +            path_len = snprintf(bp, PATH_MAX, "%s/%s", SYSFS_PATH,
> +                                ent->d_name);
> +            if (path_len < 0 || path_len >= PATH_MAX) {
> +                return false;
> +            }
> +            if (battery_verify_sysfs(s, bp)) {
> +                qdev_prop_set_string(dev, BATTERY_PATH_PROP, bp);
> +                closedir(dir);
> +                return true;
> +            }
> +        }
> +        ent = readdir(dir);
> +    }
> +    closedir(dir);
> +
> +    return false;
> +}
> +
> +static void battery_realize(DeviceState *dev, Error **errp)
> +{
> +    ISADevice *d = ISA_DEVICE(dev);
> +    BatteryState *s = BATTERY_DEVICE(dev);
> +    FWCfgState *fw_cfg = fw_cfg_find();
> +    uint16_t *battery_port;
> +    char err_details[0x20] = {};
> +
> +    trace_battery_realize();
> +
> +    if (s->use_qmp_control && s->enable_sysfs) {
> +        error_setg(errp, "Cannot enable both QMP control and sysfs mode");
> +        return;
> +    }
> +
> +    /* Initialize QMP state to sensible defaults when in QMP mode */
> +    if (s->use_qmp_control) {
> +        s->qmp_present = true;
> +        s->qmp_charging = false;
> +        s->qmp_discharging = true;
> +        s->qmp_charge_percent = 50;
> +        s->qmp_rate = 1000;  /* 1000 mW discharge rate */
> +    }
> +
> +    if (s->enable_sysfs) {
> +        if (!s->bat_path) {
> +            strcpy(err_details, " Try using 'sysfs_path='");
> +        }
> +
> +        if (!get_battery_path(dev)) {
> +            error_setg(errp, "Battery sysfs path not found or unreadable.%s",
> +                       err_details);
> +            return;
> +        }
> +        battery_get_full_charge(s, errp);
> +    } else {
> +        s->charge_full = BATTERY_FULL_CAP;
> +    }
> +
> +    isa_register_ioport(d, &s->io, s->ioport);
> +
> +    battery_probe_state_timer_init(s);
> +
> +    if (!fw_cfg) {
> +        return;
> +    }
> +
> +    battery_port = g_malloc(sizeof(*battery_port));
> +    *battery_port = cpu_to_le16(s->ioport);
> +    fw_cfg_add_file(fw_cfg, "etc/battery-port", battery_port,
> +                    sizeof(*battery_port));
> +}
> +
> +static const Property battery_device_properties[] = {
> +    DEFINE_PROP_UINT16(BATTERY_IOPORT_PROP, BatteryState, ioport, 0x530),
> +    DEFINE_PROP_BOOL("use-qmp", BatteryState, use_qmp_control, true),
> +    DEFINE_PROP_BOOL("enable-sysfs", BatteryState, enable_sysfs, false),
> +    DEFINE_PROP_UINT64(BATTERY_PROBE_STATE_INTERVAL, BatteryState,
> +                       probe_state_interval, 2000),
> +    DEFINE_PROP_STRING(BATTERY_PATH_PROP, BatteryState, bat_path),
> +};
> +
> +static const VMStateDescription battery_vmstate = {
> +    .name = "battery",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_UINT16(ioport, BatteryState),
> +        VMSTATE_UINT64(probe_state_interval, BatteryState),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
> +static void build_battery_aml(AcpiDevAmlIf *adev, Aml *scope)
> +{
> +    Aml *dev, *field, *method, *pkg;
> +    Aml *bat_state, *bat_rate, *bat_charge;
> +    Aml *sb_scope;
> +    BatteryState *s = BATTERY_DEVICE(adev);
> +
> +    bat_state  = aml_local(0);
> +    bat_rate   = aml_local(1);
> +    bat_charge = aml_local(2);
> +
> +    sb_scope = aml_scope("\\_SB");
> +    dev = aml_device("BAT0");
> +    aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0C0A")));
> +
> +    method = aml_method("_STA", 0, AML_NOTSERIALIZED);
> +    aml_append(method, aml_return(aml_int(0x1F)));
> +    aml_append(dev, method);
> +
> +    aml_append(dev, aml_operation_region("DBST", AML_SYSTEM_IO,
> +                                         aml_int(s->ioport),
> +                                         BATTERY_LEN));
> +    field = aml_field("DBST", AML_DWORD_ACC, AML_NOLOCK, AML_PRESERVE);
> +    aml_append(field, aml_named_field("BSTA", 32));
> +    aml_append(field, aml_named_field("BRTE", 32));
> +    aml_append(field, aml_named_field("BCRG", 32));
> +    aml_append(dev, field);
> +
> +    method = aml_method("_BIF", 0, AML_NOTSERIALIZED);
> +    pkg = aml_package(13);
> +    /* Power Unit */
> +    aml_append(pkg, aml_int(0));             /* mW */
> +    /* Design Capacity */
> +    aml_append(pkg, aml_int(BATTERY_FULL_CAP));
> +    /* Last Full Charge Capacity */
> +    aml_append(pkg, aml_int(BATTERY_FULL_CAP));
> +    /* Battery Technology */
> +    aml_append(pkg, aml_int(1));             /* Secondary */
> +    /* Design Voltage */
> +    aml_append(pkg, aml_int(BATTERY_VAL_UNKNOWN));
> +    /* Design Capacity of Warning */
> +    aml_append(pkg, aml_int(BATTERY_CAPACITY_OF_WARNING));
> +    /* Design Capacity of Low */
> +    aml_append(pkg, aml_int(BATTERY_CAPACITY_OF_LOW));
> +    /* Battery Capacity Granularity 1 */
> +    aml_append(pkg, aml_int(BATTERY_CAPACITY_GRANULARITY));
> +    /* Battery Capacity Granularity 2 */
> +    aml_append(pkg, aml_int(BATTERY_CAPACITY_GRANULARITY));
> +    /* Model Number */
> +    aml_append(pkg, aml_string("QBAT001"));  /* Model Number */
> +    /* Serial Number */
> +    aml_append(pkg, aml_string("SN00000"));  /* Serial Number */
> +    /* Battery Type */
> +    aml_append(pkg, aml_string("Virtual"));  /* Battery Type */
> +    /* OEM Information */
> +    aml_append(pkg, aml_string("QEMU"));     /* OEM Information */
> +    aml_append(method, aml_return(pkg));
> +    aml_append(dev, method);
> +
> +    pkg = aml_package(4);
> +    /* Battery State */
> +    aml_append(pkg, aml_int(0));
> +    /* Battery Present Rate */
> +    aml_append(pkg, aml_int(BATTERY_VAL_UNKNOWN));
> +    /* Battery Remaining Capacity */
> +    aml_append(pkg, aml_int(BATTERY_VAL_UNKNOWN));
> +    /* Battery Present Voltage */
> +    aml_append(pkg, aml_int(BATTERY_VAL_UNKNOWN));
> +    aml_append(dev, aml_name_decl("DBPR", pkg));
> +
> +    method = aml_method("_BST", 0, AML_NOTSERIALIZED);
> +    aml_append(method, aml_store(aml_name("BSTA"), bat_state));
> +    aml_append(method, aml_store(aml_name("BRTE"), bat_rate));
> +    aml_append(method, aml_store(aml_name("BCRG"), bat_charge));
> +    aml_append(method, aml_store(bat_state,
> +                                 aml_index(aml_name("DBPR"), aml_int(0))));
> +    aml_append(method, aml_store(bat_rate,
> +                                 aml_index(aml_name("DBPR"), aml_int(1))));
> +    aml_append(method, aml_store(bat_charge,
> +                                 aml_index(aml_name("DBPR"), aml_int(2))));
> +    aml_append(method, aml_return(aml_name("DBPR")));
> +    aml_append(dev, method);
> +
> +    aml_append(sb_scope, dev);
> +    aml_append(scope, sb_scope);
> +
> +    /* Device Check */
> +    method = aml_method("\\_GPE._E07", 0, AML_NOTSERIALIZED);
> +    aml_append(method, aml_notify(aml_name("\\_SB.BAT0"), aml_int(0x01)));
> +    aml_append(scope, method);
> +
> +    /* Status Change */
> +    method = aml_method("\\_GPE._E08", 0, AML_NOTSERIALIZED);
> +    aml_append(method, aml_notify(aml_name("\\_SB.BAT0"), aml_int(0x80)));
> +    aml_append(scope, method);
> +
> +    /* Information Change */
> +    method = aml_method("\\_GPE._E09", 0, AML_NOTSERIALIZED);
> +    aml_append(method, aml_notify(aml_name("\\_SB.BAT0"), aml_int(0x81)));
> +    aml_append(scope, method);
> +}
> +
> +static void battery_class_init(ObjectClass *class, const void *data)
> +{
> +    DeviceClass *dc = DEVICE_CLASS(class);
> +    AcpiDevAmlIfClass *adevc = ACPI_DEV_AML_IF_CLASS(class);
> +
> +    dc->realize = battery_realize;
> +    device_class_set_props(dc, battery_device_properties);
> +    dc->vmsd = &battery_vmstate;
> +    adevc->build_dev_aml = build_battery_aml;
> +}
> +
> +static uint64_t battery_ioport_read(void *opaque, hwaddr addr, unsigned size)
> +{
> +    BatteryState *s = opaque;
> +
> +    battery_get_dynamic_status(s);
> +
> +    switch (addr) {
> +    case bsta_addr0:
> +        return s->state.acc[0];
> +    case bsta_addr1:
> +        return s->state.acc[1];
> +    case bsta_addr2:
> +        return s->state.acc[2];
> +    case bsta_addr3:
> +        return s->state.acc[3];
> +    case brte_addr0:
> +        return s->rate.acc[0];
> +    case brte_addr1:
> +        return s->rate.acc[1];
> +    case brte_addr2:
> +        return s->rate.acc[2];
> +    case brte_addr3:
> +        return s->rate.acc[3];
> +    case bcrg_addr0:
> +        return s->charge.acc[0];
> +    case bcrg_addr1:
> +        return s->charge.acc[1];
> +    case bcrg_addr2:
> +        return s->charge.acc[2];
> +    case bcrg_addr3:
> +        return s->charge.acc[3];
> +    default:
> +        warn_report("Battery: guest read unknown value.");
> +        trace_battery_ioport_read_unknown();
> +        return 0;
> +    }
> +}
> +
> +static const MemoryRegionOps battery_ops = {
> +    .read = battery_ioport_read,
> +    .impl = {
> +        .min_access_size = 1,
> +        .max_access_size = 1,
> +    },
> +};
> +
> +static void battery_instance_init(Object *obj)
> +{
> +    BatteryState *s = BATTERY_DEVICE(obj);
> +
> +    memory_region_init_io(&s->io, obj, &battery_ops, s, "battery",
> +                          BATTERY_LEN);
> +}
> +
> +static const TypeInfo battery_info = {
> +    .name          = TYPE_BATTERY,
> +    .parent        = TYPE_ISA_DEVICE,
> +    .instance_size = sizeof(BatteryState),
> +    .class_init    = battery_class_init,
> +    .instance_init = battery_instance_init,
> +    .interfaces = (InterfaceInfo[]) {
> +        { TYPE_ACPI_DEV_AML_IF },
> +        { },
> +    },
> +};
> +
> +static BatteryState *find_battery_device(void)
> +{
> +    Object *o = object_resolve_path_type("", TYPE_BATTERY, NULL);
> +    if (!o) {
> +        return NULL;
> +    }
> +    return BATTERY_DEVICE(o);
> +}
> +
> +void qmp_battery_set_state(BatteryInfo *state, Error **errp)
> +{
> +    BatteryState *s = find_battery_device();
> +
> +    if (!s) {
> +        error_setg(errp, "No battery device found");
> +        return;
> +    }
> +
> +    s->qmp_present = state->present;
> +    s->qmp_charging = state->charging;
> +    s->qmp_discharging = state->discharging;
> +    s->qmp_charge_percent = state->charge_percent;
> +
> +    if (state->has_rate) {
> +        s->qmp_rate = state->rate;
> +    }
> +
> +    Object *obj = object_resolve_path_type("", TYPE_ACPI_DEVICE_IF, NULL);
> +    if (obj) {
> +        acpi_send_event(DEVICE(obj), ACPI_BATTERY_CHANGE_STATUS);
> +    }
> +}
> +
> +BatteryInfo *qmp_query_battery(Error **errp)
> +{
> +    BatteryState *s = find_battery_device();
> +    BatteryInfo *ret;
> +
> +    if (!s) {
> +        error_setg(errp, "No battery device found");
> +        return NULL;
> +    }
> +
> +    ret = g_new0(BatteryInfo, 1);
> +
> +    if (s->use_qmp_control) {
> +        ret->present = s->qmp_present;
> +        ret->charging = s->qmp_charging;
> +        ret->discharging = s->qmp_discharging;
> +        ret->charge_percent = s->qmp_charge_percent;
> +        ret->has_rate = true;
> +        ret->rate = s->qmp_rate;
> +    } else {
> +        battery_get_dynamic_status(s);
> +        ret->present = true;
> +        ret->charging = !!(s->state.val & BATTERY_CHARGING);
> +        ret->discharging = !!(s->state.val & BATTERY_DISCHARGING);
> +        ret->charge_percent = (s->charge.val * 100) / BATTERY_FULL_CAP;
> +        ret->has_rate = true;
> +        ret->rate = s->rate.val;
> +    }
> +
> +    ret->has_remaining_capacity = false;
> +    ret->has_design_capacity = true;
> +    ret->design_capacity = BATTERY_FULL_CAP;
> +
> +    return ret;
> +}
> +
> +static void battery_register_types(void)
> +{
> +    type_register_static(&battery_info);
> +}
> +
> +type_init(battery_register_types)
> diff --git a/hw/acpi/meson.build b/hw/acpi/meson.build
> index 73f02b9691..10379a7b2c 100644
> --- a/hw/acpi/meson.build
> +++ b/hw/acpi/meson.build
> @@ -31,6 +31,7 @@ acpi_ss.add(when: 'CONFIG_PC', if_false: files('acpi-x86-stub.c'))
>   if have_tpm
>     acpi_ss.add(files('tpm.c'))
>   endif
> +acpi_ss.add(when: 'CONFIG_BATTERY', if_true: files('battery.c'))
>   system_ss.add(when: 'CONFIG_ACPI', if_false: files('acpi-stub.c', 'aml-build-stub.c', 'ghes-stub.c', 'acpi_interface.c'))
>   system_ss.add(when: 'CONFIG_ACPI_PCI_BRIDGE', if_false: files('pci-bridge-stub.c'))
>   system_ss.add_all(when: 'CONFIG_ACPI', if_true: acpi_ss)
> diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
> index edc93e703c..dd3e815482 100644
> --- a/hw/acpi/trace-events
> +++ b/hw/acpi/trace-events
> @@ -87,3 +87,8 @@ acpi_nvdimm_read_io_port(void) "Alert: we never read _DSM IO Port"
>   acpi_nvdimm_dsm_mem_addr(uint64_t dsm_mem_addr) "dsm memory address 0x%" PRIx64
>   acpi_nvdimm_dsm_info(uint32_t revision, uint32_t handle, uint32_t function) "Revision 0x%" PRIx32 " Handle 0x%" PRIx32 " Function 0x%" PRIx32
>   acpi_nvdimm_invalid_revision(uint32_t revision) "Revision 0x%" PRIx32 " is not supported, expect 0x1"
> +
> +# battery.c
> +battery_realize(void) "Battery device realize entry"
> +battery_get_dynamic_status(uint32_t state, uint32_t rate, uint32_t charge) "Battery read state: 0x%"PRIx32", rate: %"PRIu32", charge: %"PRIu32
> +battery_ioport_read_unknown(void) "Battery read unknown"
> diff --git a/hw/i386/Kconfig b/hw/i386/Kconfig
> index 3a0e2b8ebb..2c878fd112 100644
> --- a/hw/i386/Kconfig
> +++ b/hw/i386/Kconfig
> @@ -39,6 +39,7 @@ config PC
>       imply VIRTIO_VGA
>       imply NVDIMM
>       imply FDC_ISA
> +    imply BATTERY
>       select I8259
>       select I8254
>       select PCKBD
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index 423c4959fe..790b16e582 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -1248,6 +1248,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
>   
>           aml_append(sb_scope, dev);
>       }
> +
>       aml_append(dsdt, sb_scope);
>   
>       if (pm->pcihp_bridge_en || pm->pcihp_root_en) {
> diff --git a/include/hw/acpi/acpi_dev_interface.h b/include/hw/acpi/acpi_dev_interface.h
> index 68d9d15f50..3064ef6734 100644
> --- a/include/hw/acpi/acpi_dev_interface.h
> +++ b/include/hw/acpi/acpi_dev_interface.h
> @@ -13,6 +13,7 @@ typedef enum {
>       ACPI_NVDIMM_HOTPLUG_STATUS = 16,
>       ACPI_VMGENID_CHANGE_STATUS = 32,
>       ACPI_POWER_DOWN_STATUS = 64,
> +    ACPI_BATTERY_CHANGE_STATUS = 128,
>   } AcpiEventStatusBits;
>   
>   #define TYPE_ACPI_DEVICE_IF "acpi-device-interface"
> diff --git a/include/hw/acpi/battery.h b/include/hw/acpi/battery.h
> new file mode 100644
> index 0000000000..5c5e83abfa
> --- /dev/null
> +++ b/include/hw/acpi/battery.h
> @@ -0,0 +1,33 @@
> +/*
> + * QEMU emulated battery device.
> + *
> + * Copyright (c) 2019 Janus Technologies, Inc. (http://janustech.com)
> + *
> + * Authors:
> + *     Leonid Bloch <lb.workbox@gmail.com>
> + *     Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> + *     Dmitry Fleytman <dmitry.fleytman@gmail.com>
> + *
> + * SPDX-License-Identifier: GPL-2.0-or-later
> + *
> + */
> +
> +#ifndef HW_ACPI_BATTERY_H
> +#define HW_ACPI_BATTERY_H
> +
> +#define TYPE_BATTERY                  "battery"
> +#define BATTERY_IOPORT_PROP           "ioport"
> +#define BATTERY_PATH_PROP             "sysfs_path"
> +#define BATTERY_PROBE_STATE_INTERVAL  "probe_interval"
> +
> +#define BATTERY_FULL_CAP     10000  /* mWh */
> +
> +#define BATTERY_CAPACITY_OF_WARNING   (BATTERY_FULL_CAP /  10)  /* 10% */
> +#define BATTERY_CAPACITY_OF_LOW       (BATTERY_FULL_CAP /  25)  /* 4%  */
> +#define BATTERY_CAPACITY_GRANULARITY  (BATTERY_FULL_CAP / 100)  /* 1%  */
> +
> +#define BATTERY_VAL_UNKNOWN  0xFFFFFFFF
> +
> +#define BATTERY_LEN          0x0C
> +
> +#endif
> diff --git a/qapi/acpi.json b/qapi/acpi.json
> index 906b3687a5..d1ad663bfd 100644
> --- a/qapi/acpi.json
> +++ b/qapi/acpi.json
> @@ -142,3 +142,76 @@
>   ##
>   { 'event': 'ACPI_DEVICE_OST',
>        'data': { 'info': 'ACPIOSTInfo' } }
> +
> +##
> +# @BatteryInfo:
> +#
> +# Battery state information
> +#
> +# @present: whether the battery is present
> +#
> +# @charging: whether the battery is charging
> +#
> +# @discharging: whether the battery is discharging
> +#
> +# @charge-percent: battery charge percentage (0-100)
> +#
> +# @rate: charge/discharge rate in mW (optional)
> +#
> +# @remaining-capacity: remaining capacity in mWh (optional)
> +#
> +# @design-capacity: design capacity in mWh (optional)
> +#
> +# Since: 10.2
> +##
> +{ 'struct': 'BatteryInfo',
> +  'data': { 'present': 'bool',
> +            'charging': 'bool',
> +            'discharging': 'bool',
> +            'charge-percent': 'int',
> +            '*rate': 'int',
> +            '*remaining-capacity': 'int',
> +            '*design-capacity': 'int' } }
> +
> +##
> +# @battery-set-state:
> +#
> +# Set the state of the emulated battery device
> +#
> +# @state: new battery state
> +#
> +
> +#
> +# Since: 10.2
> +#
> +# .. qmp-example::
> +#
> +#     -> { "execute": "battery-set-state",
> +#          "arguments": { "state": { "present": true,
> +#                                    "charging": true,
> +#                                    "discharging": false,
> +#                                    "charge-percent": 85 } } }
> +#     <- { "return": {} }
> +##
> +{ 'command': 'battery-set-state',
> +  'data': { 'state': 'BatteryInfo' } }
> +
> +##
> +# @query-battery:
> +#
> +# Query the current state of the emulated battery device
> +#
> +# Returns: current battery state
> +#
> +# Since: 10.2
> +#
> +# .. qmp-example::
> +#
> +#     -> { "execute": "query-battery" }
> +#     <- { "return": { "present": true,
> +#                      "charging": true,
> +#                      "discharging": false,
> +#                      "charge-percent": 85 } }
> +##
> +{ 'command': 'query-battery',
> +  'returns': 'BatteryInfo' }
The compilation is broken with this patch for non-x86 targets
without CONFIG_ACPI and CONFIG_BATTERY. Linker reports QMP
implementation routines as non-available.

The same applies for further patches in the series.

Please take a look to hw/acpi/meson.build for this line:
system_ss.add(when: 'CONFIG_ACPI', if_false: files('acpi-stub.c', 
'aml-build-stub.c', 'ghes-stub.c', 'acpi_interface.c')

Something similar is necessary.

Den