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

* [RFC PATCH 1/2] target/i386: add compatibility property for arch_capabilities
  2025-09-10  8:24 ` [RFC PATCH 0/2] Fix cross migration issue with missing features: pdcm, arch-capabilities Hector Cao
@ 2025-09-10  8:24   ` Hector Cao
  0 siblings, 0 replies; 5+ messages in thread
From: Hector Cao @ 2025-09-10  8:24 UTC (permalink / raw)
  To: grub-devel

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,
+                     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


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