Re: [RFC PATCH] char-fe: implement qemu_chr_fe_backend_name

On Mon, Aug 10, 2026 at 7:36 PM Alex Bennée <[email protected]> wrote:
>
> A number of chardev users where calling qemu_chr_fe_get_driver() which
> is discouraged as it blocks backend hotswap. As the pattern is common
> and need for the string is transitory add a helper and keep
> qemu_chr_fe_get_driver() for those that really need the funkiness of
> poking around the backend setup.
>
> Signed-off-by: Alex Bennée <[email protected]>
> Suggested-by: Peter Maydell <[email protected]>

Reviewed-by: Marc-André Lureau <[email protected]>

> ---
>  include/chardev/char-fe.h       | 13 ++++++++++++-
>  backends/cryptodev-vhost-user.c |  8 +-------
>  backends/rng-egd.c              |  8 +-------
>  backends/vhost-user.c           |  8 +-------
>  hw/char/imx_serial.c            |  4 ++--
>  5 files changed, 17 insertions(+), 24 deletions(-)
>
> diff --git a/include/chardev/char-fe.h b/include/chardev/char-fe.h
> index 5f8a6df17dc..13218d04c8d 100644
> --- a/include/chardev/char-fe.h
> +++ b/include/chardev/char-fe.h
> @@ -52,7 +52,8 @@ void qemu_chr_fe_deinit(CharFrontend *c, bool del);
>   * associated Chardev.
>   * Note: avoid this function as the driver should never be accessed directly,
>   *       especially by the frontends that support chardevice hotswap.
> - *       Consider qemu_chr_fe_backend_connected() to check for driver existence
> + *       Consider qemu_chr_fe_backend_connected() to check for driver
> + *       existence or qemu_chr_fe_backend_name() if you need the name.
>   */
>  Chardev *qemu_chr_fe_get_driver(CharFrontend *c);
>
> @@ -70,6 +71,16 @@ bool qemu_chr_fe_backend_connected(CharFrontend *c);
>   */
>  bool qemu_chr_fe_backend_open(CharFrontend *c);
>
> +/**
> +  * qemu_chr_fe_backend_name:
> +  *
> +  * Returns: caller freeable string or NULL
> +  */
> +static inline char * qemu_chr_fe_backend_name(CharFrontend *c)
> +{
> +    return (c->chr && c->chr->label) ? g_strdup(c->chr->label) : NULL;
> +}
> +
>  /**
>   * qemu_chr_fe_set_handlers_full:
>   * @c: a CharFrontend
> diff --git a/backends/cryptodev-vhost-user.c b/backends/cryptodev-vhost-user.c
> index cc478d9902d..3334e280a27 100644
> --- a/backends/cryptodev-vhost-user.c
> +++ b/backends/cryptodev-vhost-user.c
> @@ -373,13 +373,7 @@ cryptodev_vhost_user_get_chardev(Object *obj, Error **errp)
>  {
>      CryptoDevBackendVhostUser *s =
>                        CRYPTODEV_BACKEND_VHOST_USER(obj);
> -    Chardev *chr = qemu_chr_fe_get_driver(&s->chr);
> -
> -    if (chr && chr->label) {
> -        return g_strdup(chr->label);
> -    }
> -
> -    return NULL;
> +    return qemu_chr_fe_backend_name(&s->chr);
>  }
>
>  static void cryptodev_vhost_user_finalize(Object *obj)
> diff --git a/backends/rng-egd.c b/backends/rng-egd.c
> index 1d92bd71cf7..c2207f40c5c 100644
> --- a/backends/rng-egd.c
> +++ b/backends/rng-egd.c
> @@ -126,13 +126,7 @@ static void rng_egd_set_chardev(Object *obj, const char *value, Error **errp)
>  static char *rng_egd_get_chardev(Object *obj, Error **errp)
>  {
>      RngEgd *s = RNG_EGD(obj);
> -    Chardev *chr = qemu_chr_fe_get_driver(&s->chr);
> -
> -    if (chr && chr->label) {
> -        return g_strdup(chr->label);
> -    }
> -
> -    return NULL;
> +    return qemu_chr_fe_backend_name(&s->chr);
>  }
>
>  static void rng_egd_finalize(Object *obj)
> diff --git a/backends/vhost-user.c b/backends/vhost-user.c
> index 380d8250233..46dadb74800 100644
> --- a/backends/vhost-user.c
> +++ b/backends/vhost-user.c
> @@ -152,13 +152,7 @@ static void set_chardev(Object *obj, const char *value, Error **errp)
>  static char *get_chardev(Object *obj, Error **errp)
>  {
>      VhostUserBackend *b = VHOST_USER_BACKEND(obj);
> -    Chardev *chr = qemu_chr_fe_get_driver(&b->chr);
> -
> -    if (chr && chr->label) {
> -        return g_strdup(chr->label);
> -    }
> -
> -    return NULL;
> +    return qemu_chr_fe_backend_name(&b->chr);
>  }
>
>  static void vhost_user_backend_class_init(ObjectClass *oc, const void *data)
> diff --git a/hw/char/imx_serial.c b/hw/char/imx_serial.c
> index fb41ee2ac50..8ac4efbb5e9 100644
> --- a/hw/char/imx_serial.c
> +++ b/hw/char/imx_serial.c
> @@ -278,10 +278,10 @@ static void imx_serial_write(void *opaque, hwaddr offset,
>                               uint64_t value, unsigned size)
>  {
>      IMXSerialState *s = (IMXSerialState *)opaque;
> -    Chardev *chr = qemu_chr_fe_get_driver(&s->chr);
> +    g_autofree char *label = qemu_chr_fe_backend_name(&s->chr);
>      unsigned char ch;
>
> -    trace_imx_serial_write(chr ? chr->label : "NODEV", offset, value);
> +    trace_imx_serial_write(label ? label : "NODEV", offset, value);
>
>      switch (offset >> 2) {
>      case 0x10: /* UTXD */
> --
> 2.47.3
>
>