Re: [PATCH v2] s390x/cpu_model: disallow unpack for --only-migratable

[Cornelia Huck]

On Mon, 25 Jan 2021 14:22:38 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> secure execution (aka protected virtualization) guests cannot be
> migrated at the moment. Disallow the unpack facility if the user
> specifies --only-migratable.

Maybe make the explanation a bit more verbose?

"Secure execution (aka protected virtualization) guests cannot be
migrated at the moment. If the unpack facility is provided in the cpu
model, a guest may choose to transition to secure mode, making the
guest unmigratable at that point in time. If the machine was explicitly
started with --only-migratable, we would get a failure only when the
guest actually tries to transition; instead, explicitly disallow the
unpack facility if --only-migratable was specified to avoid late
surprises."

> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
> v1->v2:
> - add missing return
> - protect check with CONFIG_USER_ONLY for building non softmmu binaries
> 
>  target/s390x/cpu_models.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
> index 35179f9dc7ba..e844a4007210 100644
> --- a/target/s390x/cpu_models.c
> +++ b/target/s390x/cpu_models.c
> @@ -26,6 +26,7 @@
>  #include "qapi/qmp/qdict.h"
>  #ifndef CONFIG_USER_ONLY
>  #include "sysemu/arch_init.h"
> +#include "sysemu/sysemu.h"
>  #include "hw/pci/pci.h"
>  #endif
>  #include "qapi/qapi-commands-machine-target.h"
> @@ -878,6 +879,14 @@ static void check_compatibility(const S390CPUModel 
> *max_model,
>          return;
>      }
>  
> +#ifndef CONFIG_USER_ONLY
> +    if (only_migratable && test_bit(S390_FEAT_UNPACK, model->features)) {
> +        error_setg(errp, "The unpack facility is not compatible with "
> +                   "the --only-migratable option");

Might be a bit surprising if the host model had been specified... is
there a way to add a hint how to get rid of the unpack bit?

> +     return;
> +    }
> +#endif
> +
>      /* detect the missing features to properly report them */
>      bitmap_andnot(missing, model->features, max_model->features, 
> S390_FEAT_MAX);
>      if (bitmap_empty(missing, S390_FEAT_MAX)) {