[xen staging] CPU: abstract read-mostly-ness for per-CPU cpumask_var_t variables

[patchbot@xxxxxxx]

commit 67e055ee9c1a5a28d5fbc3c54bcab920bb9b7215
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Feb 3 15:11:16 2026 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Feb 3 15:11:16 2026 +0100

    CPU: abstract read-mostly-ness for per-CPU cpumask_var_t variables
    
    cpumask_var_t can resolve to a pointer or to an array. While the pointer
    typically is allocated once for a CPU and then only read (i.e. wants to be
    marked read-mostly), the same isn't necessarily true for the array case.
    There things depend on how the variable is actually used. cpu_core_mask
    and cpu_sibling_mask (which all architectures have inherited from x86,
    which in turn is possibly wrong) are altered only as CPUs are brought up
    or down, so may remain uniformly read-mostly. Other (x86-only) instances
    want to change, to avoid disturbing adjacent read-mostly data.
    
    While doing the x86 adjustment, also do one in the opposite direction,
    i.e. where there was no read-mostly annotation when it is applicable in
    the "pointer" case.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/genapic/x2apic.c | 2 +-
 xen/arch/x86/smpboot.c        | 6 +++---
 xen/include/xen/cpumask.h     | 6 ++++++
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/genapic/x2apic.c b/xen/arch/x86/genapic/x2apic.c
index 89d66bc627..5f2765cf14 100644
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/xen/arch/x86/genapic/x2apic.c
@@ -20,7 +20,7 @@
 static DEFINE_PER_CPU_READ_MOSTLY(u32, cpu_2_logical_apicid);
 static DEFINE_PER_CPU_READ_MOSTLY(cpumask_t *, cluster_cpus);
 static cpumask_t *cluster_cpus_spare;
-static DEFINE_PER_CPU(cpumask_var_t, scratch_mask);
+static DEFINE_PER_CPU_CPUMASK_VAR(scratch_mask);
 
 static inline u32 x2apic_cluster(unsigned int cpu)
 {
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 69cc9bbc6e..961bdf5333 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -52,13 +52,13 @@ DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_mask);
 /* representing HT and core siblings of each logical CPU */
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_mask);
 
-DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, scratch_cpumask);
+DEFINE_PER_CPU_CPUMASK_VAR(scratch_cpumask);
 static cpumask_t scratch_cpu0mask;
 
-DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, hpet_scratch_cpumask);
+DEFINE_PER_CPU_CPUMASK_VAR(hpet_scratch_cpumask);
 static cpumask_t hpet_scratch_cpu0mask;
 
-DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, send_ipi_cpumask);
+DEFINE_PER_CPU_CPUMASK_VAR(send_ipi_cpumask);
 static cpumask_t send_ipi_cpu0mask;
 
 DEFINE_PER_CPU_READ_MOSTLY(struct stubs, stubs);
diff --git a/xen/include/xen/cpumask.h b/xen/include/xen/cpumask.h
index b713bb69a9..f3a53d90e0 100644
--- a/xen/include/xen/cpumask.h
+++ b/xen/include/xen/cpumask.h
@@ -311,6 +311,9 @@ extern const cpumask_t cpumask_all;
 
 typedef cpumask_t *cpumask_var_t;
 
+#define DEFINE_PER_CPU_CPUMASK_VAR(sym) \
+       DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, sym)
+
 static inline bool alloc_cpumask_var(cpumask_var_t *mask)
 {
        *mask = _xmalloc(nr_cpumask_bits / 8, sizeof(long));
@@ -349,6 +352,9 @@ static inline void free_cpumask_var(cpumask_var_t mask)
 #else
 typedef cpumask_t cpumask_var_t[1];
 
+#define DEFINE_PER_CPU_CPUMASK_VAR(sym) \
+       DEFINE_PER_CPU(cpumask_var_t, sym)
+
 static inline bool alloc_cpumask_var(cpumask_var_t *mask)
 {
        return true;
--
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