Re: [PATCH v7 13/13] s390: Recognize confidential-guest-support option

[Christian Borntraeger]

On 15.01.21 17:36, Cornelia Huck wrote:
> On Thu, 14 Jan 2021 10:58:11 +1100
> David Gibson <david@gibson.dropbear.id.au> 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 by the "confidential-guest-support"
>> 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 confidential-guest-support 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 confidential-guest-support
>>    is set, we will either create a guest capable of entering PV mode,
>>    or fail outright.
>>
>>  - If confidential-guest-support is not set, guests 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.
>>
>> To start a guest supporting Protected Virtualization using the new
>> option use the command line arguments:
>>     -object s390-pv-guest,id=pv0 -machine confidential-guest-support=pv0
>>
>> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
>> ---
>>  docs/confidential-guest-support.txt |  3 ++
>>  docs/system/s390x/protvirt.rst      | 19 ++++++---
>>  hw/s390x/pv.c                       | 62 +++++++++++++++++++++++++++++
>>  include/hw/s390x/pv.h               |  1 +
>>  target/s390x/kvm.c                  |  3 ++
>>  5 files changed, 82 insertions(+), 6 deletions(-)
>>
> 
> (...)
> 
>> +int s390_pv_init(ConfidentialGuestSupport *cgs, Error **errp)
>> +{
>> +    if (!object_dynamic_cast(OBJECT(cgs), TYPE_S390_PV_GUEST)) {
>> +        return 0;
>> +    }
>> +
>> +    if (!s390_has_feat(S390_FEAT_UNPACK)) {
>> +        error_setg(errp,
>> +                   "CPU model does not support Protected Virtualization");
>> +        return -1;
>> +    }
>> +
>> +    cgs->ready = true;
>> +
>> +    return 0;
>> +}
> 
> Do we want to add a migration blocker here? If we keep the one that is
> added when the guest transitions, we'll end up with two of them, but
> that might be easier than trying to unify it.

that whould be fine with me. We still need to move things around to
make sure that the cpu model is already in place, though.