Re: [PATCH v2] target/i386: add compat for migrating error code

[Zhao Liu <zhao1.liu@intel.com>]

On Tue, Mar 10, 2026 at 04:43:23PM +0100, Fiona Ebner wrote:
> Date: Tue, 10 Mar 2026 16:43:23 +0100
> From: Fiona Ebner <f.ebner@proxmox.com>
> Subject: [PATCH v2] target/i386: add compat for migrating error code
> X-Mailer: git-send-email 2.47.3
> 
> 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>
> ---
> 
> Changes in v2:
> * Add proper machine version compat rather than just checking if
>   error_code != 0.
> 
>  hw/i386/pc.c          | 1 +
>  target/i386/cpu.c     | 1 +
>  target/i386/cpu.h     | 1 +
>  target/i386/machine.c | 2 +-
>  4 files changed, 4 insertions(+), 1 deletion(-)

...

> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 5a62aa6157..ecae8593c8 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2361,6 +2361,7 @@ struct ArchCPU {
>      bool expose_tcg;
>      bool migratable;
>      bool migrate_smi_count;

only a nit, it's better to have a comment like:

    /*
     * Compatibility bits for old machine types.
     * If true, allow to migrate error code.
     */

> +    bool migrate_error_code;
>      uint32_t apic_id;

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>