[v13,10/12] xen/arm: add Xen cache colors command line parameter

Commit Message

Carlo Nonato Dec. 17, 2024, 5:06 p.m. UTC

From: Luca Miccio <[e-mail]>

Add a new command line parameter to configure Xen cache colors.
These colors are dumped together with other coloring info.

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:
- Xen 1 color latency:  3.1 us
- Xen 2 color latency:  3.1 us

Since this is the most common target for Arm cache coloring, the default
amount of Xen colors is set to one.

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.

Signed-off-by: Luca Miccio <[e-mail]>
Signed-off-by: Marco Solieri <[e-mail]>
Signed-off-by: Carlo Nonato <[e-mail]>
Reviewed-by: Jan Beulich <[e-mail]>
---
v13:
- no changes
v12:
- no changes
v11:
- no changes
v10:
- no changes
v9:
- no changes
v8:
- added bound check on xen_colors in llc_coloring_init()
v7:
- removed XEN_DEFAULT_COLOR
- XEN_DEFAULT_NUM_COLORS is now used in a for loop to set xen default colors
---
 docs/misc/cache-coloring.rst      |  2 ++
 docs/misc/xen-command-line.pandoc | 10 ++++++++++
 xen/common/llc-coloring.c         | 29 +++++++++++++++++++++++++++++
 3 files changed, 41 insertions(+)

Patch

diff --git a/docs/misc/cache-coloring.rst b/docs/misc/cache-coloring.rst
index 5224b27afe..e156062aa2 100644
--- a/docs/misc/cache-coloring.rst
+++ b/docs/misc/cache-coloring.rst
@@ -115,6 +115,8 @@  Specific documentation is available at `docs/misc/xen-command-line.pandoc`.
 +----------------------+-------------------------------+
 | ``buddy-alloc-size`` | Buddy allocator reserved size |
 +----------------------+-------------------------------+
+| ``xen-llc-colors``   | Xen color configuration       |
++----------------------+-------------------------------+
 
 Colors selection format
 ***********************
diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index de5d68a452..4f1fde84df 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2925,6 +2925,16 @@  mode.
 **WARNING: `x2apic_phys` is deprecated and superseded by `x2apic-mode`.
 The latter takes precedence if both are set.**
 
+### 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/common/llc-coloring.c b/xen/common/llc-coloring.c
index bebd6e2056..0cfaa77a2b 100644
--- a/xen/common/llc-coloring.c
+++ b/xen/common/llc-coloring.c
@@ -13,6 +13,7 @@ 
 #include <xen/types.h>
 
 #define NR_LLC_COLORS          (1U << CONFIG_LLC_COLORS_ORDER)
+#define XEN_DEFAULT_NUM_COLORS 1
 
 /*
  * -1: not specified (disabled unless llc-size and llc-nr-ways present)
@@ -36,6 +37,9 @@  static unsigned int __ro_after_init default_colors[NR_LLC_COLORS];
 static unsigned int __initdata dom0_colors[NR_LLC_COLORS];
 static unsigned int __initdata dom0_num_colors;
 
+static unsigned int __ro_after_init xen_colors[NR_LLC_COLORS];
+static unsigned int __ro_after_init xen_num_colors;
+
 #define mfn_color_mask              (max_nr_colors - 1)
 #define mfn_to_color(mfn)           (mfn_x(mfn) & mfn_color_mask)
 
@@ -94,6 +98,13 @@  static int __init parse_dom0_colors(const char *s)
 }
 custom_param("dom0-llc-colors", parse_dom0_colors);
 
+static int __init parse_xen_colors(const char *s)
+{
+    return parse_color_config(s, xen_colors, ARRAY_SIZE(xen_colors),
+                              &xen_num_colors);
+}
+custom_param("xen-llc-colors", parse_xen_colors);
+
 static void print_colors(const unsigned int colors[], unsigned int num_colors)
 {
     unsigned int i;
@@ -178,6 +189,22 @@  void __init llc_coloring_init(void)
     for ( i = 0; i < max_nr_colors; i++ )
         default_colors[i] = i;
 
+    if ( !xen_num_colors )
+    {
+        unsigned int i;
+
+        xen_num_colors = MIN(XEN_DEFAULT_NUM_COLORS, max_nr_colors);
+
+        printk(XENLOG_WARNING
+               "Xen LLC color config not found. Using first %u colors\n",
+               xen_num_colors);
+        for ( i = 0; i < xen_num_colors; i++ )
+            xen_colors[i] = i;
+    }
+    else if ( xen_num_colors > max_nr_colors ||
+              !check_colors(xen_colors, xen_num_colors) )
+        panic("Bad LLC color config for Xen\n");
+
     arch_llc_coloring_init();
 }
 
@@ -188,6 +215,8 @@  void dump_llc_coloring_info(void)
 
     printk("LLC coloring info:\n");
     printk("    Number of LLC colors supported: %u\n", max_nr_colors);
+    printk("    Xen LLC colors (%u): ", xen_num_colors);
+    print_colors(xen_colors, xen_num_colors);
 }
 
 void domain_dump_llc_colors(const struct domain *d)