Re: [PATCH v5 20/23] KVM: nVMX: Add FRED VMCS fields to nested VMX context handling

[Chao Gao <chao.gao@intel.com>]

>@@ -2578,6 +2588,17 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
> 		vmcs_writel(GUEST_IDTR_BASE, vmcs12->guest_idtr_base);
> 
> 		vmx_segment_cache_clear(vmx);
>+
>+		if (nested_cpu_load_guest_fred_states(vmcs12)) {
>+			vmcs_write64(GUEST_IA32_FRED_CONFIG, vmcs12->guest_ia32_fred_config);
>+			vmcs_write64(GUEST_IA32_FRED_RSP1, vmcs12->guest_ia32_fred_rsp1);
>+			vmcs_write64(GUEST_IA32_FRED_RSP2, vmcs12->guest_ia32_fred_rsp2);
>+			vmcs_write64(GUEST_IA32_FRED_RSP3, vmcs12->guest_ia32_fred_rsp3);
>+			vmcs_write64(GUEST_IA32_FRED_STKLVLS, vmcs12->guest_ia32_fred_stklvls);
>+			vmcs_write64(GUEST_IA32_FRED_SSP1, vmcs12->guest_ia32_fred_ssp1);
>+			vmcs_write64(GUEST_IA32_FRED_SSP2, vmcs12->guest_ia32_fred_ssp2);
>+			vmcs_write64(GUEST_IA32_FRED_SSP3, vmcs12->guest_ia32_fred_ssp3);
>+		}

I think we need to snapshot L1's FRED MSR values before nested VM entry and
propagate them to GUEST_IA32_FRED* of VMCS02 for
!nested_cpu_load_guest_fred_states(vmcs12) case. i.e., from guest's view,
FRED MSRs shouldn't change across VM-entry if "Load guest FRED states" isn't
set.

Refer to the comment above 'pre_vmenter_debugctl' definition and also the
CET implmenetation*.

[*]: https://lore.kernel.org/kvm/20250704085027.182163-22-chao.gao@intel.com/