Architecturally, stepping is a 4-bit field, so a uint16_t suffices for a
bitmap of steppings.

In order to keep the size of struct x86_cpu_id the same, shrink the vendor and
family fields; there's no need for them to be uint16_t in Xen, and this
matches the size of the fields in cpuinfo_x86.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>

v2:
 * Rebase over adjustments to x86_match_cpu().
 * Switch name to X86_STEPPING_ANY, and use 0xffff to remove a conditional
   from x86_match_cpu().

Combined with the previous patch, the code generation continues to be
reasonable despite the analysis tangent taken on the v1 thread.
---
 xen/arch/x86/cpu/common.c            |  2 ++
 xen/arch/x86/include/asm/match-cpu.h | 12 ++++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index a659ea7bf604..7851142b737d 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -1022,6 +1022,8 @@ const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id table[])
             continue;
         if ( c->model != m->model )
             continue;
+        if ( !((1U << c->stepping) & m->steppings) )
+            continue;
         if ( !cpu_has(c, m->feature) )
             continue;
 
diff --git a/xen/arch/x86/include/asm/match-cpu.h b/xen/arch/x86/include/asm/match-cpu.h
index f8e4e37d9972..e25f414ea1b4 100644
--- a/xen/arch/x86/include/asm/match-cpu.h
+++ b/xen/arch/x86/include/asm/match-cpu.h
@@ -8,28 +8,32 @@
 #include <asm/intel-family.h>
 #include <asm/x86-vendors.h>
 
+#define X86_STEPPING_ANY 0xffff
 #define X86_FEATURE_ANY X86_FEATURE_LM
 
 struct x86_cpu_id {
-    uint16_t vendor;
-    uint16_t family;
+    uint8_t  vendor;
+    uint8_t  family;
     uint16_t model;
+    uint16_t steppings; /* Stepping bitmap, or X86_STEPPING_ANY */
     uint16_t feature;   /* X86_FEATURE_*, or X86_FEATURE_ANY */
     const void *driver_data;
 };
 
-#define X86_MATCH_CPU(v, f, m, feat, data)                      \
+#define X86_MATCH_CPU(v, f, m, steps, feat, data)               \
     {                                                           \
         .vendor       = (v),                                    \
         .family       = (f),                                    \
         .model        = (m),                                    \
+        .steppings    = (steps),                                \
         .feature      = (feat),                                 \
         .driver_data  = (const void *)(unsigned long)(data),    \
     }
 
 #define X86_MATCH_VFM(vfm, data)                                \
     X86_MATCH_CPU(VFM_VENDOR(vfm), VFM_FAMILY(vfm),             \
-                  VFM_MODEL(vfm), X86_FEATURE_ANY, data)
+                  VFM_MODEL(vfm), X86_STEPPING_ANY,             \
+                  X86_FEATURE_ANY, data)
 
 /*
  * x86_match_cpu() - match the CPU against an array of x86_cpu_ids[]
-- 
2.39.5

On 18.07.2025 16:20, Andrew Cooper wrote:
> Architecturally, stepping is a 4-bit field, so a uint16_t suffices for a
> bitmap of steppings.
> 
> In order to keep the size of struct x86_cpu_id the same, shrink the vendor and
> family fields; there's no need for them to be uint16_t in Xen, and this
> matches the size of the fields in cpuinfo_x86.
> 
> No functional change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

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