Re: [RFC PATCH 2/2] target/i386: add compatibility property for pdcm feature

[Avnish Chouhan <avnish@linux.ibm.com>]

On 2025-09-10 18:27, grub-devel-request@gnu.org wrote:
> Message: 3
> Date: Wed, 10 Sep 2025 10:24:32 +0200
> From: Hector Cao <hector.cao@canonical.com>
> To: grub-devel@gnu.org
> Subject: [RFC PATCH 2/2] target/i386: add compatibility property for
> 	pdcm feature
> Message-ID: <20250910082432.14764-3-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>
> ---
>  hw/core/machine.c     |  1 +
>  migration/migration.h | 11 +++++++++++
>  migration/options.c   |  3 +++
>  target/i386/cpu.c     | 17 ++++++++++++++---
>  4 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 8ad5d79cb3..535184c221 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -46,6 +46,7 @@ GlobalProperty hw_compat_10_0[] = {
>      { "ramfb", "use-legacy-x86-rom", "true"},
>      { "vfio-pci-nohotplug", "use-legacy-x86-rom", "true" },
>      { "migration", "arch-cap-always-on", "true" },
> +    { "migration", "pdcm-on-even-without-pmu", "true" },
>  };
>  const size_t hw_compat_10_0_len = G_N_ELEMENTS(hw_compat_10_0);
> 
> diff --git a/migration/migration.h b/migration/migration.h
> index 5124ff3636..7d5b2aa042 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -522,6 +522,17 @@ struct MigrationState {
>       * to a host with QEMU 10.1 with error : missing feature 
> arch-capabilities
>       */
>      bool arch_cap_always_on;
> +
> +    /*
> +     * This variable allows to keep the backward compatibility with
> QEMU (<10.1)
> +     * on the pdcm feature detection. The pdcm feature should be 
> disabled when
> +     * PMU is not available. Prio to 10.1, there is a bug and pdcm can 
> still be
> +     * enabled even if PMU is off. This behavior has been fixed by the 
> commit
> +     * e68ec29 (since 10.1).
> +     * This new behavior breaks migration of VMs that expect, with the 
> QEMU
> +     * (since 10.1), pdcm to be disabled.
> +     */
> +    bool pdcm_on_even_without_pmu;
>  };
> 
>  void migrate_set_state(MigrationStatus *state, MigrationStatus 
> old_state,
> diff --git a/migration/options.c b/migration/options.c
> index 3a80dba9c5..a2a95dfcc4 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -206,6 +206,9 @@ const Property migration_properties[] = {
> 
>      DEFINE_PROP_BOOL("arch-cap-always-on", MigrationState,
>                       arch_cap_always_on, false),
> +
> +    DEFINE_PROP_BOOL("pdcm-on-even-without-pmu", MigrationState,

Hi Hector,

Missing space before '('

> +                     pdcm_on_even_without_pmu, false),
>  };
>  const size_t migration_properties_count = 
> ARRAY_SIZE(migration_properties);
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 6d85149e6e..1f0f2c8dbf 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -51,6 +51,8 @@
>  #include "disas/capstone.h"
>  #include "cpu-internal.h"
> 
> +#include "migration/migration.h"
> +
>  static void x86_cpu_realizefn(DeviceState *dev, Error **errp);
>  static void x86_cpu_get_supported_cpuid(uint32_t func, uint32_t index,
>                                          uint32_t *eax, uint32_t *ebx,
> @@ -7839,6 +7841,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t
> index, uint32_t count,
>      uint32_t signature[3];
>      X86CPUTopoInfo *topo_info = &env->topo_info;
>      uint32_t threads_per_pkg;
> +    MigrationState *ms = migrate_get_current();
> 
>      threads_per_pkg = x86_threads_per_pkg(topo_info);
> 
> @@ -7894,6 +7897,11 @@ 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 (ms->pdcm_on_even_without_pmu) {

Please move '{' to next line.

> +            if (!cpu->enable_pmu) {

Same as above!

> +                *ecx &= ~CPUID_EXT_PDCM;
> +            }
> +        }
>          break;
>      case 2: { /* cache info: needed for Pentium Pro compatibility */
>          const CPUCaches *caches;
> @@ -8892,6 +8900,7 @@ void x86_cpu_expand_features(X86CPU *cpu, Error 
> **errp)
>      FeatureWord w;
>      int i;
>      GList *l;
> +    MigrationState *ms = migrate_get_current();
> 
>      for (l = plus_features; l; l = l->next) {
>          const char *prop = l->data;
> @@ -8944,9 +8953,11 @@ 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 (!ms->pdcm_on_even_without_pmu) {

Same as above!

> +        /* PDCM is fixed1 bit for TDX */
> +        if (!cpu->enable_pmu && !is_tdx_vm()) {

same as above!
Thank you!

Regards,
Avnish Chouhan

> +            env->features[FEAT_1_ECX] &= ~CPUID_EXT_PDCM;
> +        }
>      }
> 
>      for (i = 0; i < ARRAY_SIZE(feature_dependencies); i++) {
> --
> 2.45.2
> 
> 

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