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

[Xin Li]

On 7/23/2025 11:50 PM, Chao Gao wrote:
> > @@ -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@xxxxxxxxx/

Nice catch, it really took me a while to understand the issue.