[PATCHv10 1/2] cpu: Add x86 stepping number as another factor to find candidate CPU

[Wang Huaqiang <huaqiang.wang@xxxxxxxxx>]

Cascadelake-SP is looked as the second generation Intel XEON processor
scalable family while Skylake-SP is the first generation. Both
Skylake-SP and Cascadelake-SP has the same family (6h) and model (55h),
but with difference stepping number.

In the process of identifying candidate CPU, the stepping number is not
irrelevant any more. The CPU refresh from Skylake-SP to Cascadelake-SP
is this kind of example.

This patch add the stepping number as another factor to identify future
Intel CPU.

Signed-off-by: Wang Huaqiang <huaqiang.wang@xxxxxxxxx>
---
 src/cpu/cpu_x86.c | 60 ++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 48 insertions(+), 12 deletions(-)

diff --git a/src/cpu/cpu_x86.c b/src/cpu/cpu_x86.c
index ebfa74f..20b471b 100644
--- a/src/cpu/cpu_x86.c
+++ b/src/cpu/cpu_x86.c
@@ -615,8 +615,14 @@ x86DataToSignatureFull(const virCPUx86Data *data,
 }
 
 
-/* Mask out irrelevant bits (R and Step) from processor signature. */
-#define SIGNATURE_MASK  0x0fff3ff0
+#define SIGNATURE_MASK  0x0fff3fff
+#define SIGNATURE_MASK_STEPPING 0x0000000f
+#define SIGNATURE_MASK_FAMILYMODEL  0x0fff3ff0
+
+#define SIGNATURE(sig) (sig & SIGNATURE_MASK)
+#define STEPPING(sig) (sig & SIGNATURE_MASK_STEPPING)
+#define FAMILYMODEL(sig) (sig & SIGNATURE_MASK_FAMILYMODEL)
+
 
 static uint32_t
 x86DataToSignature(const virCPUx86Data *data)
@@ -627,7 +633,7 @@ x86DataToSignature(const virCPUx86Data *data)
     if (!(cpuid = x86DataCpuid(data, &leaf1)))
         return 0;
 
-    return cpuid->eax & SIGNATURE_MASK;
+    return SIGNATURE(cpuid->eax);
 }
 
 
@@ -1203,6 +1209,7 @@ x86ModelParse(xmlXPathContextPtr ctxt,
     if (virXPathBoolean("boolean(./signature)", ctxt)) {
         unsigned int sigFamily = 0;
         unsigned int sigModel = 0;
+        unsigned int sigStepping = 0;
         int rc;
 
         rc = virXPathUInt("string(./signature/@family)", ctxt, &sigFamily);
@@ -1221,7 +1228,12 @@ x86ModelParse(xmlXPathContextPtr ctxt,
             goto cleanup;
         }
 
-        model->signature = x86MakeSignature(sigFamily, sigModel, 0);
+        /* CPU stepping number will be used if './signature/@stepping' is present */
+        rc = virXPathUInt("string(./signature/@stepping)", ctxt, &sigStepping);
+        if (rc < 0)
+            sigStepping = 0;
+
+        model->signature = x86MakeSignature(sigFamily, sigModel, sigStepping);
     }
 
     if (virXPathBoolean("boolean(./vendor)", ctxt)) {
@@ -1675,6 +1687,13 @@ virCPUx86Compare(virCPUDefPtr host,
  * Checks whether a candidate model is a better fit for the CPU data than the
  * current model.
  *
+ * Using family/model along with an optional stepping number to select candidate
+ * CPU. If stepping is 0, consider which might be optional.
+ *
+ * If not considering the stepping number, we want to select a model with
+ * family/model equal to family/model of the real CPU. Once we found such
+ * model, we only consider candidates with matching family/model.
+ *
  * Returns 0 if current is better,
  *         1 if candidate is better,
  *         2 if candidate is the best one (search should stop now).
@@ -1707,12 +1726,22 @@ x86DecodeUseCandidate(virCPUx86ModelPtr current,
         return 1;
     }
 
-    /* Ideally we want to select a model with family/model equal to
-     * family/model of the real CPU. Once we found such model, we only
-     * consider candidates with matching family/model.
-     */
+    if (STEPPING(current->signature) && current->signature != signature) {
+        VIR_DEBUG("%s is dropped due to signature stepping mismatch, try %s",
+                cpuCurrent->model, cpuCandidate->model);
+        return 1;
+    }
+
     if (signature &&
-        current->signature == signature &&
+        FAMILYMODEL(current->signature) == FAMILYMODEL(signature) &&
+        FAMILYMODEL(candidate->signature) != FAMILYMODEL(signature)) {
+        VIR_DEBUG("%s differs in signature from matching %s",
+                  cpuCandidate->model, cpuCurrent->model);
+        return 0;
+    }
+
+    if (signature && STEPPING(candidate->signature) &&
+        FAMILYMODEL(current->signature) == FAMILYMODEL(signature) &&
         candidate->signature != signature) {
         VIR_DEBUG("%s differs in signature from matching %s",
                   cpuCandidate->model, cpuCurrent->model);
@@ -1725,12 +1754,19 @@ x86DecodeUseCandidate(virCPUx86ModelPtr current,
         return 1;
     }
 
+    if (signature && STEPPING(candidate->signature) &&
+        FAMILYMODEL(current->signature) == FAMILYMODEL(signature) &&
+        candidate->signature == signature) {
+        VIR_DEBUG("%s provides matching signature", cpuCandidate->model);
+        return 1;
+    }
+
     /* Prefer a candidate with matching signature even though it would
      * result in longer list of features.
      */
-    if (signature &&
-        candidate->signature == signature &&
-        current->signature != signature) {
+    if (signature&&
+        FAMILYMODEL(candidate->signature) == FAMILYMODEL(signature) &&
+        FAMILYMODEL(current->signature) != FAMILYMODEL(signature)) {
         VIR_DEBUG("%s provides matching signature", cpuCandidate->model);
         return 1;
     }
-- 
2.7.4

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