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

Paul Mackerras
2019-11-13T00:14:27Z

On Tue, Nov 12, 2019 at 06:45:55AM -0800, Ram Pai wrote:
> On Tue, Nov 12, 2019 at 10:32:04PM +1100, Paul Mackerras wrote:
> > On Mon, Nov 11, 2019 at 11:52:15PM -0800, Ram Pai wrote:
> > > 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.
> >
> > Most of the checks that look at whether a VM is a secure VM use code
> > like "if (kvm->arch.secure_guest & KVMPPC_SECURE_INIT_DONE)". Now
> > since KVMPPC_SECURE_INIT_ABORT is 4, an if statement such as that will
> > take the false branch once we have set kvm->arch.secure_guest to
> > KVMPPC_SECURE_INIT_ABORT in kvmppc_h_svm_init_abort. So in fact in
> > most places we will treat the VM as a normal VM from then on. If
> > there are any places where we still need to treat the VM as a secure
> > VM while we are processing the abort we can easily do that too.
>
> Is the suggestion -- KVMPPC_SECURE_INIT_ABORT should never return back
> to the Ultravisor? Because removing that assembly code will NOT lead the

No. The suggestion is that vcpu->arch.secure_guest stays set to
KVMPPC_SECURE_INIT_ABORT until userspace calls KVM_PPC_SVM_OFF.

> Hypervisor back into the Ultravisor. This is fine with the
> ultravisor. But then the hypervisor will not know where to return to.
> If it wants to return directly to the VM, it wont know to
> which address. It will be in a limbo.
>
> >
> > > 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.
> >
> > If bad things happen in the ultravisor because the hypervisor did
> > something it shouldn't, then it's game over, you just lost, thanks for
> > playing. The ultravisor MUST be able to cope with bogus UV_RETURN
> > calls for a VM that it doesn't consider to be a secure VM. You need
> > to work out how to handle such calls safely and implement that in the
> > ultravisor.
>
> Actually we do handle this gracefully in the ultravisor :).
> We just retun back to the hypervisor saying "sorry dont know what
> to do with it, please handle it yourself".
>
> However hypervisor would not know what to do with that return, and bad
> things happen in the hypervisor.

Right. We need something after the "sc 2" to handle the case where
the ultravisor returns with an error from the UV_RETURN.

Paul.