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

[dmkhn@proton.me]

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@ford.com>
---
Link to the original change:
- https://lore.kernel.org/xen-devel/20250103-vuart-ns8250-v3-v1-15-c5d36b31d66c@ford.com/
---
 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