Re: [PATCH v7 07/13] confidential guest support: Introduce cgs "ready" flag

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jan 18, 2021 at 07:47:30PM +0000, Dr. David Alan Gilbert wrote:
> * David Gibson (david@gibson.dropbear.id.au) wrote:
> > The platform specific details of mechanisms for implementing
> > confidential guest support may require setup at various points during
> > initialization.  Thus, it's not really feasible to have a single cgs
> > initialization hook, but instead each mechanism needs its own
> > initialization calls in arch or machine specific code.
> > 
> > However, to make it harder to have a bug where a mechanism isn't
> > properly initialized under some circumstances, we want to have a
> > common place, relatively late in boot, where we verify that cgs has
> > been initialized if it was requested.
> > 
> > This patch introduces a ready flag to the ConfidentialGuestSupport
> > base type to accomplish this, which we verify just before the machine
> > specific initialization function.
> 
> You may find you need to define 'ready' and the answer might be a bit
> variable;

Ugh, yeah.  I'm not sure it will be possible to precisely define
this.  Basically this is a helper for an internal sanity check, rather
than anything more fundamental.  The idea is that depending on the
details of the CGS mechanism, it might need initialization of various
components, which might naturally be distributed across different
parts of the init code.

> for example, on SEV there's a setup bit and then you may end
> up doing an attestation and receiving some data before you actaully let
> the guest execute code.   Is it ready before it's received the
> attestation response or only when it can run code?

I don't really know enough about how SEV works to answer that.
Certainly it should be "ready" before any guest instructions are
executed.

> Is a Power or 390 machine 'ready' before it's executed the magic
> instruction to enter the confidential mode?

Definitely.  Ready must be set before we enter the guest (and so
assert that it *is* set is before that).

> 
> Dave
> 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  hw/core/machine.c                         | 8 ++++++++
> >  include/exec/confidential-guest-support.h | 2 ++
> >  target/i386/sev.c                         | 2 ++
> >  3 files changed, 12 insertions(+)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index 94194ab82d..5a7433332b 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -1190,6 +1190,14 @@ void machine_run_board_init(MachineState *machine)
> >      }
> >  
> >      if (machine->cgs) {
> > +        /*
> > +         * Where confidential guest support is initialized depends on
> > +         * the specific mechanism in use.  But, we need to make sure
> > +         * it's ready by now.  If it isn't, that's a bug in the
> > +         * implementation of that cgs mechanism.
> > +         */
> > +        assert(machine->cgs->ready);
> > +
> >          /*
> >           * With confidential guests, the host can't see the real
> >           * contents of RAM, so there's no point in it trying to merge
> > diff --git a/include/exec/confidential-guest-support.h b/include/exec/confidential-guest-support.h
> > index 5f131023ba..bcaf6c9f49 100644
> > --- a/include/exec/confidential-guest-support.h
> > +++ b/include/exec/confidential-guest-support.h
> > @@ -27,6 +27,8 @@ OBJECT_DECLARE_SIMPLE_TYPE(ConfidentialGuestSupport, CONFIDENTIAL_GUEST_SUPPORT)
> >  
> >  struct ConfidentialGuestSupport {
> >      Object parent;
> > +
> > +    bool ready;
> >  };
> >  
> >  typedef struct ConfidentialGuestSupportClass {
> > diff --git a/target/i386/sev.c b/target/i386/sev.c
> > index e2b41ef342..3d94635397 100644
> > --- a/target/i386/sev.c
> > +++ b/target/i386/sev.c
> > @@ -737,6 +737,8 @@ int sev_kvm_init(ConfidentialGuestSupport *cgs, Error **errp)
> >      qemu_add_machine_init_done_notifier(&sev_machine_done_notify);
> >      qemu_add_vm_change_state_handler(sev_vm_state_change, sev);
> >  
> > +    cgs->ready = true;
> > +
> >      return 0;
> >  err:
> >      sev_guest = NULL;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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