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....@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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