Re: [PATCH v7 26/26] KVM: nVMX: Enable CET support for nested guest

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2023-12-06 at 17:22 +0800, Yang, Weijiang wrote:
> On 12/5/2023 6:12 PM, Maxim Levitsky wrote:
> > On Mon, 2023-12-04 at 16:50 +0800, Yang, Weijiang wrote:
> 
> [...]
> 
> > > > >    	vmx->nested.force_msr_bitmap_recalc = false;
> > > > > @@ -2469,6 +2491,18 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
> > > > >    		if (kvm_mpx_supported() && vmx->nested.nested_run_pending &&
> > > > >    		    (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS))
> > > > >    			vmcs_write64(GUEST_BNDCFGS, vmcs12->guest_bndcfgs);
> > > > > +
> > > > > +		if (vmx->nested.nested_run_pending &&
> > > > I don't think that nested.nested_run_pending check is needed.
> > > > prepare_vmcs02_rare is not going to be called unless the nested run is pending.
> > > But there're other paths along to call prepare_vmcs02_rare(), e.g., vmx_set_nested_state()-> nested_vmx_enter_non_root_mode()-> prepare_vmcs02_rare(), especially when L1 instead of L2 was running. In this case, nested.nested_run_pending == false,
> > > we don't need to update vmcs02's fields at the point until L2 is being resumed.
> > - If we restore VM from migration stream when L2 is *not running*, then prepare_vmcs02_rare won't be called,
> > because nested_vmx_enter_non_root_mode will not be called, because in turn there is no nested vmcs to load.
> > 
> > - If we restore VM from migration stream when L2 is *about to run* (KVM emulated the VMRESUME/VMLAUNCH,
> > but we didn't do the actual hardware VMLAUNCH/VMRESUME on vmcs02, then the 'nested_run_pending' will be true, it will be restored
> > from the migration stream.
> > 
> > - If we migrate while nested guest was run once but didn't VMEXIT to L1 yet, then yes, nested.nested_run_pending will be false indeed,
> > but we still need to setup vmcs02, otherwise it will be left with default zero values.
> 
> Thanks a lot for recapping these cases! I overlooked some nested flags before. It makes sense to remove nested.nested_run_pending.
> > Remember that prior to setting nested state the VM wasn't running even once usually, unlike when the guest enters nested state normally.
> > 
> > > > > +		    (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)) {
> > > > > +			if (guest_can_use(&vmx->vcpu, X86_FEATURE_SHSTK)) {
> > > > > +				vmcs_writel(GUEST_SSP, vmcs12->guest_ssp);
> > > > > +				vmcs_writel(GUEST_INTR_SSP_TABLE,
> > > > > +					    vmcs12->guest_ssp_tbl);
> > > > > +			}
> > > > > +			if (guest_can_use(&vmx->vcpu, X86_FEATURE_SHSTK) ||
> > > > > +			    guest_can_use(&vmx->vcpu, X86_FEATURE_IBT))
> > > > > +				vmcs_writel(GUEST_S_CET, vmcs12->guest_s_cet);
> > > > > +		}
> > > > >    	}
> > > > >    
> > > > >    	if (nested_cpu_has_xsaves(vmcs12))
> > > > > @@ -4300,6 +4334,15 @@ static void sync_vmcs02_to_vmcs12_rare(struct kvm_vcpu *vcpu,
> > > > >    	vmcs12->guest_pending_dbg_exceptions =
> > > > >    		vmcs_readl(GUEST_PENDING_DBG_EXCEPTIONS);
> > > > >    
> > > > > +	if (guest_can_use(&vmx->vcpu, X86_FEATURE_SHSTK)) {
> > > > > +		vmcs12->guest_ssp = vmcs_readl(GUEST_SSP);
> > > > > +		vmcs12->guest_ssp_tbl = vmcs_readl(GUEST_INTR_SSP_TABLE);
> > > > > +	}
> > > > > +	if (guest_can_use(&vmx->vcpu, X86_FEATURE_SHSTK) ||
> > > > > +	    guest_can_use(&vmx->vcpu, X86_FEATURE_IBT)) {
> > > > > +		vmcs12->guest_s_cet = vmcs_readl(GUEST_S_CET);
> > > > > +	}
> > > > The above code should be conditional on VM_ENTRY_LOAD_CET_STATE - if the guest (L2) state
> > > > was loaded, then it must be updated on exit - this is usually how VMX works.
> > > I think this is not for L2 VM_ENTRY_LOAD_CET_STATE, it happens in prepare_vmcs02_rare(). IIUC, the guest registers will be saved into VMCS fields unconditionally when vm-exit happens,
> > > so these fields for L2 guest should be synced to L1 unconditionally.
> > "the guest registers will be saved into VMCS fields unconditionally"
> > This is not true, unless there is a bug.
> 
> I checked the latest SDM, there's no such kind of wording regarding CET entry/exit control bits. The wording comes from
> the individual CET spec.:
> "10.6 VM Exit
> On processors that support CET, the VM exit saves the state of IA32_S_CET, SSP and IA32_INTERRUPT_SSP_TABLE_ADDR MSR to the VMCS guest-state area unconditionally."
> But since it doesn't appear in SDM, I shouldn't take it for granted.

SDM spec from September 2023:

28.3.1 Saving Control Registers, Debug Registers, and MSRs

"If the processor supports the 1-setting of the “load CET” VM-entry control, the contents of the IA32_S_CET and
IA32_INTERRUPT_SSP_TABLE_ADDR MSRs are saved into the corresponding fields. On processors that do not
support Intel 64 architecture, bits 63:32 of these MSRs are not saved."

Honestly it's not 100% clear if the “load CET” should be set to 1 to trigger the restore, or that this control just needs to be
supported on the CPU.
It does feel like you are right here, that CPU always saves the guest state, but allows to not load it on VM entry via 
“load CET” VM entry control.

IMHO its best to check what the bare metal does by rigging a test by patching the host kernel to not set the 'load CET' control,
and see if the CPU still updates the guest CET fields on the VM exit.


> 
> > the vmcs12 VM_ENTRY_LOAD_CET_STATE should be passed through as is to vmcs02, so if the nested guest doesn't set this bit
> > the entry/exit using vmcs02 will not touch the CET state, which is unusual but allowed by the spec I think - a nested hypervisor can opt for example to save/load
> > this state manually or use msr load/store lists instead.
> 
> Right although the use case should be rare, will modify the code to check VM_ENTRY_LOAD_CET_STATE. Thanks!


> > Regardless of this,
> > if the guest didn't set VM_ENTRY_LOAD_CET_STATE, then vmcs12 guest fields should neither be loaded on VM entry (copied to vmcs02) nor updated on VM exit,
> > (that is copied back to vmcs12) this is what is written in the VMX spec.
> 
> What's the VMX spec. your're referring to here?

SDM.

In fact, now that I am thinking about this again, it should be OK to unconditionally copy the CET fields from vmcs12 to vmcs02, because as long as the 
VM_ENTRY_LOAD_CET_STATE is not set, the CPU should care about their values in the vmcs02.

And about the other way around, assuming that I made a mistake as I said above, then the other way around is indeed unconditional.


Sorry for a bit of a confusion.

Best regards,
	Maxim Levitsky


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