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target/i386: add compat for migrating error code

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If cpu->env.has_error_code is true, backwards migration of a VM from
a QEMU binary with commit 27535e9cca to a QEMU binary without commit
27535e9cca will fail:

> kvm: error while loading state for instance 0x0 of device 'cpu'

In practice, wrongly setting the error code to 0 on the target is
often unproblematic, so additionally checking error_code != 0 in
cpu_errcode_needed() is not enough to mitigate the issue. Instead, add
proper machine version compat handling.

Cc: qemu-stable@nongnu.org
Fixes: 27535e9cca ("target/i386: Add support for save/load of exception error code")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20260310154348.495332-1-f.ebner@proxmox.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Fiona Ebner
2026-03-10 16:43:23 +01:00
committed by Paolo Bonzini
parent f4107cbe80
commit 1b80f1009d
4 changed files with 4 additions and 1 deletions
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1
hw/i386/pc.c
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@@ -78,6 +78,7 @@ const size_t pc_compat_10_2_len = G_N_ELEMENTS(pc_compat_10_2);
GlobalProperty pc_compat_10_1[] = {
{ "mch", "extended-tseg-mbytes", "16" },
{ TYPE_X86_CPU, "x-migrate-error-code", "false" },
};
const size_t pc_compat_10_1_len = G_N_ELEMENTS(pc_compat_10_1);

1
target/i386/cpu.c
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@@ -10651,6 +10651,7 @@ static const Property x86_cpu_properties[] = {
DEFINE_PROP_BOOL("tcg-cpuid", X86CPU, expose_tcg, true),
DEFINE_PROP_BOOL("x-migrate-smi-count", X86CPU, migrate_smi_count,
true),
DEFINE_PROP_BOOL("x-migrate-error-code", X86CPU, migrate_error_code, true),
/*
* lecacy_cache defaults to true unless the CPU model provides its
* own cache information (see x86_cpu_load_def()).

1
target/i386/cpu.h
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@@ -2361,6 +2361,7 @@ struct ArchCPU {
bool expose_tcg;
bool migratable;
bool migrate_smi_count;
bool migrate_error_code;
uint32_t apic_id;
/* Enables publishing of TSC increment and Local APIC bus frequencies to

2
target/i386/machine.c
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@@ -466,7 +466,7 @@ static bool cpu_errcode_needed(void *opaque)
{
X86CPU *cpu = opaque;
return cpu->env.has_error_code != 0;
return cpu->env.has_error_code != 0 && cpu->migrate_error_code;
}
static const VMStateDescription vmstate_error_code = {