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

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

On 2025-09-10 18:27, grub-devel-request@gnu.org wrote:
> Message: 2
> Date: Wed, 10 Sep 2025 10:24:31 +0200
> From: Hector Cao <hector.cao@canonical.com>
> To: grub-devel@gnu.org
> Subject: [RFC PATCH 1/2] target/i386: add compatibility property for
> 	arch_capabilities
> Message-ID: <20250910082432.14764-2-hector.cao@canonical.com>
> 
> 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
> 
> 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,

Hi Hector,

Missing space before '('

> +                     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))) {

Please move '{' in the next line.

Thank you!

Regards,
Avnish Chouhan

>              ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
>          }
>      } else if (function == 7 && index == 1 && reg == R_EAX) {
> --
> 2.45.2
> 
> 
> 
> 
> ------------------------------

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