[RFT PATCH v2 2/2] target/i386: add compatibility property for pdcm feature

[Paolo Bonzini <pbonzini@redhat.com>]

From: Hector Cao <hector.cao@canonical.com>

The pdcm feature is supposed to be disabled when PMU is not
available. Up until v10.1, pdcm feature is enabled even when PMU
is off. This behavior has been fixed but this change breaks the
migration of VMs that are run with QEMU < 10.0 and expect the pdcm
feature to be enabled on the destination host.

This commit restores the legacy behavior for machines with version
prior to 10.1 to allow the migration from older QEMU to QEMU 10.1.

Signed-off-by: Hector Cao <hector.cao@canonical.com>
Link: https://lore.kernel.org/r/20250910115733.21149-3-hector.cao@canonical.com
Fixes: e68ec298090 ("i386/cpu: Move adjustment of CPUID_EXT_PDCM before feature_dependencies[] check", 2025-06-20)
[Move property from migration object to CPU. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/cpu.h |  6 ++++++
 hw/i386/pc.c      |  1 +
 target/i386/cpu.c | 15 ++++++++++++---
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 414ca968e84..42168f1d6d8 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -2320,6 +2320,12 @@ struct ArchCPU {
      */
     bool arch_cap_always_on;
 
+    /*
+     * Backwards compatibility with QEMU <10.1. The PDCM feature is now disabled when
+     * PMU is not available, but prior to 10.1 it was enabled even if PMU is off.
+     */
+    bool pdcm_on_even_without_pmu;
+
     /* Number of physical address bits supported */
     uint32_t phys_bits;
 
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index d7f48150fdd..4668918746e 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -88,6 +88,7 @@ GlobalProperty pc_compat_10_0[] = {
     { TYPE_X86_CPU, "x-consistent-cache", "false" },
     { TYPE_X86_CPU, "x-vendor-cpuid-only-v2", "false" },
     { TYPE_X86_CPU, "x-arch-cap-always-on", "true" },
+    { TYPE_X86_CPU, "x-pdcm-on-even-without-pmu", "true" },
 };
 const size_t pc_compat_10_0_len = G_N_ELEMENTS(pc_compat_10_0);
 
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index fe369bb1284..ab18de894e4 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7908,6 +7908,11 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
             /* Fixup overflow: max value for bits 23-16 is 255. */
             *ebx |= MIN(num, 255) << 16;
         }
+        if (cpu->pdcm_on_even_without_pmu) {
+            if (!cpu->enable_pmu) {
+                *ecx &= ~CPUID_EXT_PDCM;
+            }
+        }
         break;
     case 2: { /* cache info: needed for Pentium Pro compatibility */
         const CPUCaches *caches;
@@ -8958,9 +8963,11 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
         }
     }
 
-    /* PDCM is fixed1 bit for TDX */
-    if (!cpu->enable_pmu && !is_tdx_vm()) {
-        env->features[FEAT_1_ECX] &= ~CPUID_EXT_PDCM;
+    if (!cpu->pdcm_on_even_without_pmu) {
+        /* PDCM is fixed1 bit for TDX */
+        if (!cpu->enable_pmu && !is_tdx_vm()) {
+            env->features[FEAT_1_ECX] &= ~CPUID_EXT_PDCM;
+        }
     }
 
     for (i = 0; i < ARRAY_SIZE(feature_dependencies); i++) {
@@ -10021,6 +10028,8 @@ static const Property x86_cpu_properties[] = {
 
     DEFINE_PROP_BOOL("x-arch-cap-always-on", X86CPU,
                      arch_cap_always_on, false),
+    DEFINE_PROP_BOOL("x-pdcm-on-even-without-pmu", X86CPU,
+                     pdcm_on_even_without_pmu, false),
 };
 
 #ifndef CONFIG_USER_ONLY
-- 
2.51.0