Re: [PATCH v8 7/7] xen/console: make console buffer size configurable

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 27 Jul 2026, dmukhin@xxxxxxxx wrote:
> From: Denis Mukhin <dmukhin@xxxxxxxx> 
> 
> Add new CONRING_SHIFT Kconfig parameter to specify the boot console
> buffer size as a power of 2.
> 
> The supported range is [14..27] -> [16KiB..128MiB].
> 
> Set default to 15 (32 KiB).
> 
> Update the documentation for 'conring_size=' command line option.
> 
> Resolves: https://gitlab.com/xen-project/xen/-/issues/185
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
> ---
> Changes since v7:
> - n/a
> ---
>  docs/misc/xen-command-line.pandoc |  8 ++++++--
>  xen/drivers/char/Kconfig          | 21 +++++++++++++++++++++
>  xen/drivers/char/console.c        |  6 +++---
>  3 files changed, 30 insertions(+), 5 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index 2be8772b329a..448c9bdb8254 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -425,10 +425,14 @@ The following are examples of correct specifications:
>  ### conring_size
>  > `= <size>`
>  
> -> Default: `conring_size=16k`
> -
>  Specify the size of the console ring buffer.
>  
> +The default console ring buffer size is selected at build-time via
> +`CONFIG_CONRING_SHIFT` setting.
> +
> +The run-time console ring buffer size is the maximum of the build-time value
> +and the value specified by the `conring_size=` command-line option.
> +
>  ### console
>  > `= List of [ vga | com1[H,L] | com2[H,L] | pv | dbgp | ehci | xhci | none 
> ]`
>  
> diff --git a/xen/drivers/char/Kconfig b/xen/drivers/char/Kconfig
> index 8e49a52c735b..a40a9929132b 100644
> --- a/xen/drivers/char/Kconfig
> +++ b/xen/drivers/char/Kconfig
> @@ -95,6 +95,27 @@ config SERIAL_TX_BUFSIZE
>  
>         Default value is 32768 (32KiB).
>  
> +config CONRING_SHIFT
> +     int "Console ring buffer size (power of 2)"
> +     range 14 27

anything above 20 would fail to build on arm


> +     default 15

this is OK but is double than the previous default and would be nice to
keep a note about it in xen-command-line.pandoc


> +     help
> +       Select the boot console ring buffer size as a power of 2.
> +
> +       The run-time console ring buffer is the maximum of the build-time
> +       value and the value specified by the `conring_size=` command-line
> +       option.
> +
> +       If `conring_size=` is not specified on the command line, the run-time
> +       console ring buffer size is the maximum of this value and
> +       `num_present_cpus() << (9 + xenlog_lower_thresh)`.
> +
> +         27 => 128 MiB
> +         26 =>  64 MiB
> +         ...
> +         15 =>  32 KiB (default)
> +         14 =>  16 KiB
> +
>  config XHCI
>       bool "XHCI DbC UART driver"
>       depends on X86
> diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
> index a1b8e5f5b507..76367c1dd705 100644
> --- a/xen/drivers/char/console.c
> +++ b/xen/drivers/char/console.c
> @@ -340,12 +340,12 @@ static void cf_check do_dec_thresh(unsigned char key, 
> bool unused)
>   * ********************************************************
>   */
>  
> -/* conring_size: allows a larger console ring than default (16kB). */
> +/* conring_size: override build-time CONFIG_CONRING_SHIFT setting. */
>  static unsigned int __initdata opt_conring_size;
>  size_param("conring_size", opt_conring_size);
>  
> -#define _CONRING_SIZE 16384
> -#define CONRING_IDX_MASK(i) ((i)&(conring_size-1))
> +#define _CONRING_SIZE       (1U << CONFIG_CONRING_SHIFT)
> +#define CONRING_IDX_MASK(i) ((i) & (conring_size - 1))
>  static char __initdata _conring[_CONRING_SIZE];
>  static char *__ro_after_init conring = _conring;
>  static unsigned int __ro_after_init conring_size = _CONRING_SIZE;
> -- 
> 2.54.0
>