On 2025-09-10 18:27, [email protected] wrote:
Message: 2
Date: Wed, 10 Sep 2025 10:24:31 +0200
From: Hector Cao <[email protected]>
To: [email protected]
Subject: [RFC PATCH 1/2] target/i386: add compatibility property for
arch_capabilities
Message-ID: <[email protected]>
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 <[email protected]>
---
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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