[PATCH v5 11/15] KVM: nVMX: Set UMIP bit CR4_FIXED1 MSR when emulating UMIP

Sean Christopherson
2022-06-08T01:29:26Z

Make UMIP an "allowed-1" bit CR4_FIXED1 MSR when KVM is emulating UMIP.
KVM emulates UMIP for both L1 and L2, and so should enumerate that L2 is
allowed to have CR4.UMIP=1. Not setting the bit doesn't immediately
break nVMX, as KVM does set/clear the bit in CR4_FIXED1 in response to a
guest CPUID update, i.e. KVM will correctly (dis)allow nested VM-Entry
based on whether or not UMIP is exposed to L1.

That said, KVM should enumerate the bit as being allowed from time zero,
e.g. userspace will see the wrong value if the MSR is read before CPUID
is written. And a future patch will quirk KVM's behavior of stuffing
CR4_FIXED1 in response to guest CPUID changes, as CR4_FIXED1 is not
strictly required to match the CPUID model exposed to L1.

Fixes: 0367f205a3b7 ("KVM: vmx: add support for emulating UMIP")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/nested.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index dc2f9b06b99a..5533c2474128 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6781,6 +6781,9 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
rdmsrl(MSR_IA32_VMX_CR0_FIXED1, msrs->cr0_fixed1);
rdmsrl(MSR_IA32_VMX_CR4_FIXED1, msrs->cr4_fixed1);

+ if (vmx_umip_emulated())
+ msrs->cr4_fixed1 |= X86_CR4_UMIP;
+
msrs->vmcs_enum = nested_vmx_calc_vmcs_enum_msr();
}

--
2.36.1.255.ge46751e96f-goog