target/i386: add compatibility property for pdcm feature

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>
(cherry picked from commit 6529f31e0dccadb532c80b36e3efe7aef83f9cad)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 2504365bc2ae38f8a080316178f7db64d77db1fe..0861e329b576dbe9f2a85a4fa2e81fc75f774484 100644 (file)
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -85,6 +85,7 @@ 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 fe369bb128486cbc468f3f8c6add65f2b8ffaec1..ab18de894e40a4e9eb4a53a74c7665bf262ed85c 100644 (file)
--- 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

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 414ca968e8490c47155b1e95dfd7f7c2f345ef12..42168f1d6d8b8f4dfbf7af62f796a3f932722cd6 100644 (file)
--- 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;