Re: [PATCH v10 7/8] KVM: PPC: Implement H_SVM_INIT_ABORT hcall

[Ram Pai <linuxram@us.ibm.com>]

On Tue, Nov 12, 2019 at 04:38:36PM +1100, Paul Mackerras wrote:
> On Mon, Nov 11, 2019 at 05:01:58PM -0800, Ram Pai wrote:
> > On Mon, Nov 11, 2019 at 03:19:24PM +1100, Paul Mackerras wrote:
> > > On Mon, Nov 04, 2019 at 09:47:59AM +0530, Bharata B Rao wrote:
> > > > From: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
> > > > 
> > > > Implement the H_SVM_INIT_ABORT hcall which the Ultravisor can use to
> > > > abort an SVM after it has issued the H_SVM_INIT_START and before the
> > > > H_SVM_INIT_DONE hcalls. This hcall could be used when Ultravisor
> > > > encounters security violations or other errors when starting an SVM.
> > > > 
> > > > Note that this hcall is different from UV_SVM_TERMINATE ucall which
> > > > is used by HV to terminate/cleanup an SVM.
> > > > 
> > > > In case of H_SVM_INIT_ABORT, we should page-out all the pages back to
> > > > HV (i.e., we should not skip the page-out). Otherwise the VM's pages,
> > > > possibly including its text/data would be stuck in secure memory.
> > > > Since the SVM did not go secure, its MSR_S bit will be clear and the
> > > > VM wont be able to access its pages even to do a clean exit.
> > > 
...skip...
> > 
> > If the ultravisor cleans up the SVM's state on its side and then informs
> > the Hypervisor to abort the SVM, the hypervisor will not be able to
> > cleanly terminate the VM.  Because to terminate the SVM, the hypervisor
> > still needs the services of the Ultravisor. For example: to get the
> > pages back into the hypervisor if needed. Another example is, the
> > hypervisor can call UV_SVM_TERMINATE.  Regardless of which ucall
> > gets called, the ultravisor has to hold on to enough state of the
> > SVM to service that request.
> 
> OK, that's a good reason.  That should be explained in the commit
> message.
> 
> > The current design assumes that the hypervisor explicitly informs the
> > ultravisor, that it is done with the SVM, through the UV_SVM_TERMINATE
> > ucall. Till that point the Ultravisor must to be ready to service any
> > ucalls made by the hypervisor on the SVM's behalf.
> 
> I see that UV_SVM_TERMINATE is done when the VM is being destroyed (at
> which point kvm->arch.secure_guest doesn't matter any more), and in
> kvmhv_svm_off(), where kvm->arch.secure_guest gets cleared
> explicitly.  Hence I don't see any need for clearing it in the
> assembly code on the next secure guest entry.  I think the change to
> book3s_hv_rmhandlers.S can just be dropped.

There is subtle problem removing that code from the assembly.

If the H_SVM_INIT_ABORT hcall returns to the ultravisor without clearing
kvm->arch.secure_guest, the hypervisor will continue to think that the
VM is a secure VM.   However the primary reason the H_SVM_INIT_ABORT
hcall was invoked, was to inform the Hypervisor that it should no longer
consider the VM as a Secure VM. So there is a inconsistency there.

This is fine, as long as the VM does not invoke any hcall or does not
receive any hypervisor-exceptions.  The moment either of those happen,
the control goes into the hypervisor, the hypervisor services
the exception/hcall and while returning, it will see that the
kvm->arch.secure_guest flag is set and **incorrectly** return
to the ultravisor through a UV_RETURN ucall.  Ultravisor will
not know what to do with it, because it does not consider that
VM as a Secure VM.  Bad things happen.

( Sidenote: when H_SVM_INIT_ABORT hcalls returns from the hypervisor,
  the ultravisor cleans up its internal state corresponding of that
  aborted-SVM and returns back to the caller with MSR[S]=0 )


RP