Re: [for-6.0 v5 11/13] spapr: PEF: prevent migration

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 19 Jan 2021 09:28:22 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 18.01.21 18:39, Dr. David Alan Gilbert wrote:
> > * David Gibson (david@gibson.dropbear.id.au) wrote:  
> >> On Thu, Jan 14, 2021 at 11:25:17AM +0000, Daniel P. Berrangé wrote:  
> >>> On Wed, Jan 13, 2021 at 12:42:26PM +0000, Dr. David Alan Gilbert wrote:  
> >>>> * Cornelia Huck (cohuck@redhat.com) wrote:  
> >>>>> On Tue, 5 Jan 2021 12:41:25 -0800
> >>>>> Ram Pai <linuxram@us.ibm.com> wrote:
> >>>>>  
> >>>>>> On Tue, Jan 05, 2021 at 11:56:14AM +0100, Halil Pasic wrote:  
> >>>>>>> On Mon, 4 Jan 2021 10:40:26 -0800
> >>>>>>> Ram Pai <linuxram@us.ibm.com> wrote:  
> >>>>>  
> >>>>>>>> The main difference between my proposal and the other proposal is...
> >>>>>>>>
> >>>>>>>>   In my proposal the guest makes the compatibility decision and acts
> >>>>>>>>   accordingly.  In the other proposal QEMU makes the compatibility
> >>>>>>>>   decision and acts accordingly. I argue that QEMU cannot make a good
> >>>>>>>>   compatibility decision, because it wont know in advance, if the guest
> >>>>>>>>   will or will-not switch-to-secure.
> >>>>>>>>     
> >>>>>>>
> >>>>>>> You have a point there when you say that QEMU does not know in advance,
> >>>>>>> if the guest will or will-not switch-to-secure. I made that argument
> >>>>>>> regarding VIRTIO_F_ACCESS_PLATFORM (iommu_platform) myself. My idea
> >>>>>>> was to flip that property on demand when the conversion occurs. David
> >>>>>>> explained to me that this is not possible for ppc, and that having the
> >>>>>>> "securable-guest-memory" property (or whatever the name will be)
> >>>>>>> specified is a strong indication, that the VM is intended to be used as
> >>>>>>> a secure VM (thus it is OK to hurt the case where the guest does not
> >>>>>>> try to transition). That argument applies here as well.    
> >>>>>>
> >>>>>> As suggested by Cornelia Huck, what if QEMU disabled the
> >>>>>> "securable-guest-memory" property if 'must-support-migrate' is enabled?
> >>>>>> Offcourse; this has to be done with a big fat warning stating
> >>>>>> "secure-guest-memory" feature is disabled on the machine.
> >>>>>> Doing so, will continue to support guest that do not try to transition.
> >>>>>> Guest that try to transition will fail and terminate themselves.  
> >>>>>
> >>>>> Just to recap the s390x situation:
> >>>>>
> >>>>> - We currently offer a cpu feature that indicates secure execution to
> >>>>>   be available to the guest if the host supports it.
> >>>>> - When we introduce the secure object, we still need to support
> >>>>>   previous configurations and continue to offer the cpu feature, even
> >>>>>   if the secure object is not specified.
> >>>>> - As migration is currently not supported for secured guests, we add a
> >>>>>   blocker once the guest actually transitions. That means that
> >>>>>   transition fails if --only-migratable was specified on the command
> >>>>>   line. (Guests not transitioning will obviously not notice anything.)
> >>>>> - With the secure object, we will already fail starting QEMU if
> >>>>>   --only-migratable was specified.
> >>>>>
> >>>>> My suggestion is now that we don't even offer the cpu feature if
> >>>>> --only-migratable has been specified. For a guest that does not want to
> >>>>> transition to secure mode, nothing changes; a guest that wants to
> >>>>> transition to secure mode will notice that the feature is not available
> >>>>> and fail appropriately (or ultimately, when the ultravisor call fails).
> >>>>> We'd still fail starting QEMU for the secure object + --only-migratable
> >>>>> combination.
> >>>>>
> >>>>> Does that make sense?  
> >>>>
> >>>> It's a little unusual; I don't think we have any other cases where
> >>>> --only-migratable changes the behaviour; I think it normally only stops
> >>>> you doing something that would have made it unmigratable or causes
> >>>> an operation that would make it unmigratable to fail.  
> >>>
> >>> I agree,  --only-migratable is supposed to be a *behavioural* toggle
> >>> for QEMU. It must /not/ have any impact on the guest ABI.
> >>>
> >>> A management application needs to be able to add/remove --only-migratable
> >>> at will without changing the exposing guest ABI.  
> >>
> >> At the qemu level, it sounds like the right thing to do is to fail
> >> outright if all of the below are true:
> >>  1. --only-migratable is specified
> >>  2. -cpu host is specified
> >>  3. unpack isn't explicitly disabled
> >>  4. the host CPU actually does have the unpack facility
> >>
> >> That can be changed if & when migration support is added for PV.  
> > 
> > That sounds right to me.  
> 
> as startup will fail anyway if the guest cpu model enables unpack, but the host
> cpu does not support it this can be simplified to forbid startup in qemu if
> --only-migratable is combined with unpack being active in the guest cpu model.
> 
> This is actually independent from this patch set.

Yep, I think we should just go ahead and fix this.

>  maybe just
> something like
> 
> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
> index 35179f9dc7ba..3b85ff4e31b2 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 +879,11 @@ static void check_compatibility(const S390CPUModel *max_model,
>          return;
>      }
>  
> +    if (only_migratable && test_bit(S390_FEAT_UNPACK, model->features)) {
> +        error_setg(errp, "The unpack facility is not compatible with "
> +                  "the --only-migratable option");
> +    }
> +
>      /* 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)) {
> 
> 

Want to send this as a proper patch?