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

[dmukhin@ford.com]

From: Denis Mukhin <dmukhin@ford.com> 

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@ford.com>
---
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
+	default 15
+	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