Re: [for-5.2 v4 10/10] s390: Recognize host-trust-limitation option

[Janosch Frank]

On 7/24/20 4:57 AM, David Gibson wrote:
> At least some s390 cpu models support "Protected Virtualization" (PV),
> a mechanism to protect guests from eavesdropping by a compromised
> hypervisor.
> 
> This is similar in function to other mechanisms like AMD's SEV and
> POWER's PEF, which are controlled bythe "host-trust-limitation"
> machine option.  s390 is a slightly special case, because we already
> supported PV, simply by using a CPU model with the required feature
> (S390_FEAT_UNPACK).
> 
> To integrate this with the option used by other platforms, we
> implement the following compromise:
> 
>  - When the host-trust-limitation option is set, s390 will recognize
>    it, verify that the CPU can support PV (failing if not) and set
>    virtio default options necessary for encrypted or protected guests,
>    as on other platforms.  i.e. if host-trust-limitation is set, we
>    will either create a guest capable of entering PV mode, or fail
>    outright
> 
>  - If host-trust-limitation is not set, guest's might still be able to
>    enter PV mode, if the CPU has the right model.  This may be a
>    little surprising, but shouldn't actually be harmful.

As I already explained, they have to continue to work without any change
to the VM's configuration.

Our users already expect PV to work without HTL. This feature is already
being used and the documentation has been online for a few months. I've
already heard enough complains because users found small errors in our
documentation. I'm not looking forward to complains because suddenly we
need to specify new command line arguments depending on the QEMU version.

@Cornelia: QEMU is not my expertise, am I missing something here?

> 
> To start a guest supporting Protected Virtualization using the new
> option use the command line arguments:
>     -object s390-pv-guest,id=pv0 -machine host-trust-limitation=pv0
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/s390x/pv.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
> 
> diff --git a/hw/s390x/pv.c b/hw/s390x/pv.c
> index ab3a2482aa..4bf3b345b6 100644
> --- a/hw/s390x/pv.c
> +++ b/hw/s390x/pv.c
> @@ -14,8 +14,11 @@
>  #include <linux/kvm.h>
>  
>  #include "cpu.h"
> +#include "qapi/error.h"
>  #include "qemu/error-report.h"
>  #include "sysemu/kvm.h"
> +#include "qom/object_interfaces.h"
> +#include "exec/host-trust-limitation.h"
>  #include "hw/s390x/ipl.h"
>  #include "hw/s390x/pv.h"
>  
> @@ -111,3 +114,61 @@ void s390_pv_inject_reset_error(CPUState *cs)
>      /* Report that we are unable to enter protected mode */
>      env->regs[r1 + 1] = DIAG_308_RC_INVAL_FOR_PV;
>  }
> +
> +#define TYPE_S390_PV_GUEST "s390-pv-guest"
> +#define S390_PV_GUEST(obj)                              \
> +    OBJECT_CHECK(S390PVGuestState, (obj), TYPE_S390_PV_GUEST)
> +
> +typedef struct S390PVGuestState S390PVGuestState;
> +
> +/**
> + * S390PVGuestState:
> + *
> + * The S390PVGuestState object is basically a dummy used to tell the
> + * host trust limitation system to use s390's PV mechanism.  guest.
> + *
> + * # $QEMU \
> + *         -object s390-pv-guest,id=pv0 \
> + *         -machine ...,host-trust-limitation=pv0
> + */
> +struct S390PVGuestState {
> +    Object parent_obj;
> +};
> +
> +static int s390_pv_kvm_init(HostTrustLimitation *gmpo, Error **errp)
> +{
> +    if (!s390_has_feat(S390_FEAT_UNPACK)) {
> +        error_setg(errp,
> +                   "CPU model does not support Protected Virtualization");
> +        return -1;
> +    }
> +
> +    return 0;
> +}
> +
> +static void s390_pv_guest_class_init(ObjectClass *oc, void *data)
> +{
> +    HostTrustLimitationClass *gmpc = HOST_TRUST_LIMITATION_CLASS(oc);
> +
> +    gmpc->kvm_init = s390_pv_kvm_init;
> +}
> +
> +static const TypeInfo s390_pv_guest_info = {
> +    .parent = TYPE_OBJECT,
> +    .name = TYPE_S390_PV_GUEST,
> +    .instance_size = sizeof(S390PVGuestState),
> +    .class_init = s390_pv_guest_class_init,
> +    .interfaces = (InterfaceInfo[]) {
> +        { TYPE_HOST_TRUST_LIMITATION },
> +        { TYPE_USER_CREATABLE },
> +        { }
> +    }
> +};
> +
> +static void
> +s390_pv_register_types(void)
> +{
> +    type_register_static(&s390_pv_guest_info);
> +}
> +
> +type_init(s390_pv_register_types);
>

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