Re: [PATCH v5 6/6] xen/console: add conring buffer size alignment setting

[Jan Beulich <jbeulich@suse.com>]

On 05.02.2026 02:36, dmukhin@xen.org wrote:
> From: Denis Mukhin <dmukhin@ford.com> 
> 
> Introduce CONFIG_CONRING_ALIGN_PAGE_SIZE to control rounding down of the
> user-defined conring buffer size.

What's wrong with the rounding? The more that, with the original behavior
properly restored in patch 5, it'll be a power-of-2 multiple of PAGE_SIZE
anyway?

> Also, update the logline reporting the final conring buffer size to report
> bytes instead of kilobytes, since the user-defined size may not necessarily
> be kilobyte-alined.

Yet making the number harder to grok.

> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>

Having talked to him, I don't think he meant what you're doing here. All he
apparently meant is to stop using alloc_*heap_pages(), which needlessly
supplies order-aligned memory.

> --- a/xen/drivers/char/Kconfig
> +++ b/xen/drivers/char/Kconfig
> @@ -95,6 +95,13 @@ config SERIAL_TX_BUFSIZE
>  
>  	  Default value is 32768 (32KiB).
>  
> +config CONRING_ALIGN_PAGE_SIZE
> +	bool
> +	default y

IOW "def_bool y". But what's the point of the option when there's no prompt?

> --- a/xen/drivers/char/console.c
> +++ b/xen/drivers/char/console.c
> @@ -470,12 +470,15 @@ void __init console_init_ring(void)
>          return;
>  
>      opt_conring_size = max(opt_conring_size, conring_size);
> -    size = ROUNDDOWN(opt_conring_size, PAGE_SIZE);
> -    if ( size != opt_conring_size )
> +    if ( IS_ENABLED(CONFIG_CONRING_ALIGN_PAGE_SIZE) )
>      {
> -        opt_conring_size = size;
> -        printk(XENLOG_WARNING "Rounding down console ring size to multiple of %lu KiB.\n",
> -               PAGE_SIZE >> 10);
> +        size = ROUNDDOWN(opt_conring_size, PAGE_SIZE);
> +        if ( size != opt_conring_size )
> +        {
> +            opt_conring_size = size;
> +            printk(XENLOG_WARNING "Rounding down console ring size to multiple of %lu KiB.\n",

This line wants splitting at the start of the string literal, and the full
stop wants removing if already it is being touched.

Jan