Re: [PATCH 1/2] target/i386: add compatibility property for arch_capabilities

["Daniel P. Berrangé" <berrange@redhat.com>]

CC Paolo as maintainer

On Wed, Sep 10, 2025 at 01:57:32PM +0200, Hector Cao wrote:
> Prior to v10.1, if requested by user, arch-capabilities is always on
> despite the fact that CPUID advertises it to be off/unvailable.
> this causes a migration issue for VMs that are run on a machine
> without arch-capabilities and expect this feature to be present
> on the destination host with QEMU 10.1.
> 
> This commit add a compatibility property to restore the legacy
> behavior for all machines with version prior to 10.1
>

Can you add a 'Fixes: <hash>' line to refer to the orignial
commit in 10.1 that introduced the regression.

> Signed-off-by: Hector Cao <hector.cao@canonical.com>
> ---
>  hw/core/machine.c     |  1 +
>  migration/migration.h | 12 ++++++++++++
>  migration/options.c   |  3 +++
>  target/i386/kvm/kvm.c |  5 ++++-
>  4 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 38c949c4f2..8ad5d79cb3 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -45,6 +45,7 @@ GlobalProperty hw_compat_10_0[] = {
>      { "vfio-pci", "x-migration-load-config-after-iter", "off" },
>      { "ramfb", "use-legacy-x86-rom", "true"},
>      { "vfio-pci-nohotplug", "use-legacy-x86-rom", "true" },
> +    { "migration", "arch-cap-always-on", "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 01329bf824..5124ff3636 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -510,6 +510,18 @@ struct MigrationState {
>      bool rdma_migration;
>  
>      GSource *hup_source;
> +
> +    /*
> +     * This variable allows to keep the backward compatibility with QEMU (<10.1)
> +     * on the arch-capabilities detection.
> +     * With the commit d3a2413 (since 10.1), the arch-capabilities feature is gated
> +     * with the CPUID bit (CPUID_7_0_EDX_ARCH_CAPABILITIES) instead of being always
> +     * enabled when user requests for it. this new behavior breaks migration of VMs
> +     * created and run with older QEMU on machines without IA32_ARCH_CAPABILITIES MSR,
> +     * those VMs might have arch-capabilities enabled and break when migrating
> +     * to a host with QEMU 10.1 with error : missing feature arch-capabilities
> +     */
> +    bool arch_cap_always_on;
>  };
>  
>  void migrate_set_state(MigrationStatus *state, MigrationStatus old_state,
> diff --git a/migration/options.c b/migration/options.c
> index 4e923a2e07..3a80dba9c5 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -203,6 +203,9 @@ const Property migration_properties[] = {
>                          MIGRATION_CAPABILITY_SWITCHOVER_ACK),
>      DEFINE_PROP_MIG_CAP("x-dirty-limit", MIGRATION_CAPABILITY_DIRTY_LIMIT),
>      DEFINE_PROP_MIG_CAP("mapped-ram", MIGRATION_CAPABILITY_MAPPED_RAM),
> +
> +    DEFINE_PROP_BOOL("arch-cap-always-on", MigrationState,
> +                     arch_cap_always_on, false),
>  };
>  const size_t migration_properties_count = ARRAY_SIZE(migration_properties);
>  
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 306430a052..e2ec4e6de5 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -42,6 +42,7 @@
>  #include "xen-emu.h"
>  #include "hyperv.h"
>  #include "hyperv-proto.h"
> +#include "migration/migration.h"
>  
>  #include "gdbstub/enums.h"
>  #include "qemu/host-utils.h"
> @@ -438,6 +439,7 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
>      uint32_t ret = 0;
>      uint32_t cpuid_1_edx, unused;
>      uint64_t bitmask;
> +    MigrationState *ms = migrate_get_current();
>  
>      cpuid = get_supported_cpuid(s);
>  
> @@ -508,7 +510,8 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
>           * mcahines at all, do not show the fake ARCH_CAPABILITIES MSR that
>           * KVM sets up.
>           */
> -        if (!has_msr_arch_capabs || !(edx & CPUID_7_0_EDX_ARCH_CAPABILITIES)) {
> +        if (!has_msr_arch_capabs
> +            || (!(edx & CPUID_7_0_EDX_ARCH_CAPABILITIES) && (!ms->arch_cap_always_on))) {
>              ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
>          }
>      } else if (function == 7 && index == 1 && reg == R_EAX) {
> -- 
> 2.45.2
> 
> 

With regards,
Daniel
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