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@xxxxxxxxxxxxx> > > > > > > > > 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
