[v4,10/11] xen/arm: add Xen cache colors command line parameter

Commit Message

Carlo Nonato Jan. 23, 2023, 3:47 p.m. UTC

From: Luca Miccio <[e-mail]>

This commit adds a new command line parameter to configure Xen cache
colors. These colors can be dumped with the cache coloring info debug-key.

By default, Xen uses the first color.
Benchmarking the VM interrupt response time provides an estimation of
LLC usage by Xen's most latency-critical runtime task. Results on Arm
Cortex-A53 on Xilinx Zynq UltraScale+ XCZU9EG show that one color, which
reserves 64 KiB of L2, is enough to attain best responsiveness.

More colors are instead very likely to be needed on processors whose L1
cache is physically-indexed and physically-tagged, such as Cortex-A57.
In such cases, coloring applies to L1 also, and there typically are two
distinct L1-colors. Therefore, reserving only one color for Xen would
senselessly partitions a cache memory that is already private, i.e.
underutilize it. The default amount of Xen colors is thus set to one.

Signed-off-by: Luca Miccio <[e-mail]>
Signed-off-by: Marco Solieri <[e-mail]>
Signed-off-by: Carlo Nonato <[e-mail]>
---
 docs/misc/xen-command-line.pandoc | 10 ++++++++++
 xen/arch/arm/llc_coloring.c       | 30 ++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

Patch

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index a89c0cef61..d486946648 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2796,6 +2796,16 @@  In the case that x2apic is in use, this option switches between physical and
 clustered mode.  The default, given no hint from the **FADT**, is cluster
 mode.
 
+### xen-llc-colors (arm64)
+> `= List of [ <integer> | <integer>-<integer> ]`
+
+> Default: `0: the lowermost color`
+
+Specify Xen LLC color configuration. This options is available only when
+`CONFIG_LLC_COLORING` is enabled.
+Two colors are most likely needed on platforms where private caches are
+physically indexed, e.g. the L1 instruction cache of the Arm Cortex-A57.
+
 ### xenheap_megabytes (arm32)
 > `= <size>`
 
diff --git a/xen/arch/arm/llc_coloring.c b/xen/arch/arm/llc_coloring.c
index 22612d455b..745e93a61a 100644
--- a/xen/arch/arm/llc_coloring.c
+++ b/xen/arch/arm/llc_coloring.c
@@ -19,6 +19,10 @@ 
 #include <asm/processor.h>
 #include <asm/sysregs.h>
 
+/* By default Xen uses the lowest color */
+#define XEN_DEFAULT_COLOR       0
+#define XEN_DEFAULT_NUM_COLORS  1
+
 bool llc_coloring_enabled;
 boolean_param("llc-coloring", llc_coloring_enabled);
 
@@ -33,6 +37,9 @@  static paddr_t __ro_after_init addr_col_mask;
 static unsigned int __ro_after_init dom0_colors[CONFIG_NR_LLC_COLORS];
 static unsigned int __ro_after_init dom0_num_colors;
 
+static unsigned int __ro_after_init xen_colors[CONFIG_NR_LLC_COLORS];
+static unsigned int __ro_after_init xen_num_colors;
+
 #define addr_to_color(addr) (((addr) & addr_col_mask) >> PAGE_SHIFT)
 
 /*
@@ -83,6 +90,12 @@  static int parse_color_config(const char *buf, unsigned int *colors,
     return *s ? -EINVAL : 0;
 }
 
+static int __init parse_xen_colors(const char *s)
+{
+    return parse_color_config(s, xen_colors, &xen_num_colors);
+}
+custom_param("xen-llc-colors", parse_xen_colors);
+
 static int __init parse_dom0_colors(const char *s)
 {
     return parse_color_config(s, dom0_colors, &dom0_num_colors);
@@ -166,6 +179,8 @@  static void dump_coloring_info(unsigned char key)
     printk("LLC way size: %u KiB\n", llc_way_size >> 10);
     printk("Number of LLC colors supported: %u\n", nr_colors);
     printk("Address to LLC color mask: 0x%lx\n", addr_col_mask);
+    printk("Xen LLC colors: ");
+    print_colors(xen_colors, xen_num_colors);
 }
 
 bool __init llc_coloring_init(void)
@@ -202,6 +217,21 @@  bool __init llc_coloring_init(void)
 
     addr_col_mask = (nr_colors - 1) << PAGE_SHIFT;
 
+    if ( !xen_num_colors )
+    {
+        printk(XENLOG_WARNING
+               "Xen LLC color config not found. Using default color: %u\n",
+               XEN_DEFAULT_COLOR);
+        xen_colors[0] = XEN_DEFAULT_COLOR;
+        xen_num_colors = XEN_DEFAULT_NUM_COLORS;
+    }
+
+    if ( !check_colors(xen_colors, xen_num_colors) )
+    {
+        printk(XENLOG_ERR "Bad LLC color config for Xen\n");
+        return false;
+    }
+
     register_keyhandler('K', dump_coloring_info, "dump LLC coloring info", 1);
 
     return true;