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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 12 Feb 2025, dmkhn@xxxxxxxxx wrote:
> Add new CONRING_SIZE Kconfig parameter to specify the boot console buffer size
> in bytes. The value is rounded to the nearest power of 2 to match existing
> conring_size= behavior.
> 
> The supported range is [16KiB..128MiB].
> 
> Bump default size to 32 KiB.
> 
> Link: https://gitlab.com/xen-project/xen/-/issues/185
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> Link to the original change:
> - 
> https://lore.kernel.org/xen-devel/20250103-vuart-ns8250-v3-v1-15-c5d36b31d66c@xxxxxxxx/
> ---
>  docs/misc/xen-command-line.pandoc |  5 ++++-
>  xen/drivers/char/Kconfig          | 12 ++++++++++++
>  xen/drivers/char/console.c        |  6 +++---
>  3 files changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index 9bbd00baef..563cdbdd49 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -425,10 +425,13 @@ The following are examples of correct specifications:
>  ### conring_size
>  > `= <size>`
>  
> -> Default: `conring_size=16k`
> +> Default: `conring_size=32k`
>  
>  Specify the size of the console ring buffer.
>  
> +The console ring buffer size can be selected at build time via
> +CONFIG_CONRING_SIZE.
> +
>  ### 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 e6e12bb413..54a3a79da3 100644
> --- a/xen/drivers/char/Kconfig
> +++ b/xen/drivers/char/Kconfig
> @@ -96,6 +96,18 @@ config SERIAL_TX_BUFSIZE
>  
>         Default value is 32768 (32KiB).
>  
> +config CONRING_SIZE
> +     int "Console buffer size"
> +     default 32768
> +     range 16384 134217728
> +     help
> +       Select the boot console buffer size (in bytes).
> +       Note, the value provided will be rounded down to the nearest power of 
> 2.
> +       Run-time console buffer size is the same as the boot console size,
> +       unless overridden via 'conring_size=' boot parameter.
> +
> +       Default value is 32768 (32KiB).
> +
>  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 ee5f720de4..73d24a7821 100644
> --- a/xen/drivers/char/console.c
> +++ b/xen/drivers/char/console.c
> @@ -101,12 +101,12 @@ static int cf_check parse_console_timestamps(const char 
> *s);
>  custom_runtime_param("console_timestamps", parse_console_timestamps,
>                       con_timestamp_mode_upd);
>  
> -/* conring_size: allows a large console ring than default (16kB). */
> +/* conring_size: override build-time CONRING_SIZE setting. */
>  static uint32_t __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       (1UL << (31 - 
> __builtin_clz(CONFIG_CONRING_SIZE)))
> +#define CONRING_IDX_MASK(i) ((i) & (conring_size - 1))
>  static char __initdata _conring[_CONRING_SIZE];
>  static char *__read_mostly conring = _conring;
>  static uint32_t __read_mostly conring_size = _CONRING_SIZE;
> -- 
> 2.34.1
> 
>