[PATCH v6 1/4] xen/console: make console buffer size configurable

[dmukhin@xxxxxxxx]

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).

Resolves: https://gitlab.com/xen-project/xen/-/issues/185
Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
Changes since v5:
- shortened the Kconfig option description
- kept Jason's R-b since the change is minimal - just dropping
  few lines from the Kconfig description
---
 docs/misc/xen-command-line.pandoc |  5 +++--
 xen/drivers/char/Kconfig          | 15 +++++++++++++++
 xen/drivers/char/console.c        |  6 +++---
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 6c77129732bf..29393631d885 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -425,10 +425,11 @@ 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.
+
 ### 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..11f48415c12a 100644
--- a/xen/drivers/char/Kconfig
+++ b/xen/drivers/char/Kconfig
@@ -95,6 +95,21 @@ config SERIAL_TX_BUFSIZE
 
          Default value is 32768 (32KiB).
 
+config CONRING_SHIFT
+       int "Console ring buffer size (power of 2)"
+       range 14 27
+       default 15
+       help
+         Select the boot console ring buffer size as a power of 2.
+         Run-time console ring buffer size is the same as the boot console ring
+         buffer size, unless overridden via 'conring_size=' boot parameter.
+
+           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 bcd6d261491b..522b2f489a53 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -339,12 +339,12 @@ 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 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       (1U << CONFIG_CONRING_SHIFT)
+#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;
-- 
2.54.0