Right now, domains start with no colouring information and depend on a
XEN_DOMCTL_set_llc_colors(nr=0) hypercall in order to get default colours.

This in turn forces the toolstack to make a hypercall for all VMs even those
without colouring configured, and to ignore errors if Xen doesn't have
colouring active.

Introduce domain_llc_coloring_init(), replacing domain_set_default_colors(),
and call it during the trivial initialisation in domain_create().  Leave two
comments explaining the positioning.

The -EEXISTs condition in domain_set_llc_colors() needs adjusting as a
consequence, but that's easy enough to keep working as before.

Fixes: 6cdea3444eaf ("xen/arm: add Dom0 cache coloring support")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Anthony PERARD <anthony.perard@vates.tech>
CC: Michal Orzel <michal.orzel@amd.com>
CC: Jan Beulich <jbeulich@suse.com>
CC: Julien Grall <julien@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Carlo Nonato <carlo.nonato@minervasys.tech>
CC: Marco Solieri <marco.solieri@minervasys.tech>
---
 xen/common/domain.c            |  6 ++++++
 xen/common/llc-coloring.c      | 33 ++++++++++++++-------------------
 xen/include/xen/llc-coloring.h |  2 ++
 3 files changed, 22 insertions(+), 19 deletions(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 4f79ba39878c..28c2cc78cd88 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -878,6 +878,12 @@ struct domain *domain_create(domid_t domid,
     rwlock_init(&d->pci_lock);
 #endif
 
+    /*
+     * Doesn't allocate memory itself, but does set up data relevant for
+     * memory allocations.
+     */
+    domain_llc_coloring_init(d);
+
     /* All error paths can depend on the above setup. */
 
     /*
diff --git a/xen/common/llc-coloring.c b/xen/common/llc-coloring.c
index ea3e0ca07017..77a54beed1bf 100644
--- a/xen/common/llc-coloring.c
+++ b/xen/common/llc-coloring.c
@@ -230,24 +230,12 @@ void domain_dump_llc_colors(const struct domain *d)
     print_colors(d->llc_colors, d->num_llc_colors);
 }
 
-static void domain_set_default_colors(struct domain *d)
-{
-    printk(XENLOG_WARNING
-           "LLC color config not found for %pd, using all colors\n", d);
-
-    d->llc_colors = default_colors;
-    d->num_llc_colors = max_nr_colors;
-}
-
 int __init dom0_set_llc_colors(struct domain *d)
 {
     typeof(*dom0_colors) *colors;
 
     if ( !dom0_num_colors )
-    {
-        domain_set_default_colors(d);
         return 0;
-    }
 
     if ( (dom0_num_colors > max_nr_colors) ||
          !check_colors(dom0_colors, dom0_num_colors) )
@@ -272,14 +260,11 @@ int domain_set_llc_colors(struct domain *d,
 {
     unsigned int *colors;
 
-    if ( d->num_llc_colors )
+    if ( d->llc_colors != default_colors )
         return -EEXIST;
 
     if ( !config->num_llc_colors )
-    {
-        domain_set_default_colors(d);
         return 0;
-    }
 
     if ( config->num_llc_colors > max_nr_colors )
         return -EINVAL;
@@ -307,6 +292,19 @@ int domain_set_llc_colors(struct domain *d,
     return 0;
 }
 
+void domain_llc_coloring_init(struct domain *d)
+{
+    if ( !llc_coloring_enabled )
+        return;
+
+    /*
+     * Any change to this logic needs to consider the position of our call in
+     * domain_create().
+     */
+    d->llc_colors = default_colors;
+    d->num_llc_colors = max_nr_colors;
+}
+
 void domain_llc_coloring_free(struct domain *d)
 {
     d->num_llc_colors = 0;
@@ -321,10 +319,7 @@ int __init domain_set_llc_colors_from_str(struct domain *d, const char *str)
     unsigned int *colors, num_colors;
 
     if ( !str )
-    {
-        domain_set_default_colors(d);
         return 0;
-    }
 
     colors = xmalloc_array(unsigned int, max_nr_colors);
     if ( !colors )
diff --git a/xen/include/xen/llc-coloring.h b/xen/include/xen/llc-coloring.h
index 45f250f9f39d..94dd22e949e7 100644
--- a/xen/include/xen/llc-coloring.h
+++ b/xen/include/xen/llc-coloring.h
@@ -21,6 +21,7 @@ extern bool llc_coloring_enabled;
 void llc_coloring_init(void);
 void dump_llc_coloring_info(void);
 void domain_dump_llc_colors(const struct domain *d);
+void domain_llc_coloring_init(struct domain *d);
 void domain_llc_coloring_free(struct domain *d);
 #else
 #define llc_coloring_enabled false
@@ -28,6 +29,7 @@ void domain_llc_coloring_free(struct domain *d);
 static inline void llc_coloring_init(void) {}
 static inline void dump_llc_coloring_info(void) {}
 static inline void domain_dump_llc_colors(const struct domain *d) {}
+static inline void domain_llc_coloring_init(struct domain *d) {}
 static inline void domain_llc_coloring_free(struct domain *d) {}
 #endif
 
-- 
2.39.5

On 24.07.2025 18:23, Andrew Cooper wrote:
> Right now, domains start with no colouring information and depend on a
> XEN_DOMCTL_set_llc_colors(nr=0) hypercall in order to get default colours.
> 
> This in turn forces the toolstack to make a hypercall for all VMs even those
> without colouring configured, and to ignore errors if Xen doesn't have
> colouring active.
> 
> Introduce domain_llc_coloring_init(), replacing domain_set_default_colors(),
> and call it during the trivial initialisation in domain_create().  Leave two
> comments explaining the positioning.
> 
> The -EEXISTs condition in domain_set_llc_colors() needs adjusting as a
> consequence, but that's easy enough to keep working as before.
> 
> Fixes: 6cdea3444eaf ("xen/arm: add Dom0 cache coloring support")
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>
preferably with ...

> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -878,6 +878,12 @@ struct domain *domain_create(domid_t domid,
>      rwlock_init(&d->pci_lock);
>  #endif
>  
> +    /*
> +     * Doesn't allocate memory itself, but does set up data relevant for
> +     * memory allocations.
> +     */
> +    domain_llc_coloring_init(d);

... the word "default" added to the comment here.

Jan