Re: [PATCH v2 2/2] KVM: nVMX: Remove explicit filtering of GUEST_INTR_STATUS from shadow VMCS fields

[Sean Christopherson]

On Wed, Dec 31, 2025, Chao Gao wrote:
> On Tue, Dec 30, 2025 at 02:02:20PM -0800, Sean Christopherson wrote:
> >Drop KVM's filtering of GUEST_INTR_STATUS when generating the shadow VMCS
> >bitmap now that KVM drops GUEST_INTR_STATUS from the set of supported
> >vmcs12 fields if the field isn't supported by hardware.
> 
> IIUC, the construction of the shadow VMCS bitmap and fields doesn't reference
> "the set of supported vmcs12 fields".

Argh, right you are.  I assumed init_vmcs_shadow_fields() would already verify
the field is a valid vmcs12 field, at least as a sanity check, but it doesn't.

> So, with the filtering dropped, copy_shadow_to_vmcs12() and
> copy_vmcs12_to_shadow() may access GUEST_INTR_STATUS on unsupported hardware.
> 
> Do we need something like this (i.e., don't shadow unsupported vmcs12 fields)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index f50d21a6a2d7..08433b3713d2 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -127,6 +127,8 @@ static void init_vmcs_shadow_fields(void)
> 				continue;
> 			break;
> 		default:
> +			if (!cpu_has_vmcs12_field(field))

This can be

			if (get_vmcs12_field_offset(field) < 0)

And I think I'll put it outside the switch statement, because the requirement
applies to all fields, even those that have additional restrictions.

I also think it makes sense to have patch 1 call nested_vmx_setup_vmcs12_fields()
from nested_vmx_hardware_setup(), so that the ordering and dependency between
configuring vmcs12 fields and shadow VMCS fields can be explicitly documented.