Re: [RFC PATCH v6 06/36] KVM: nVMX: Treat eVMCS as enabled for guest iff Hyper-V is also enabled

Vitaly Kuznetsov
2022-08-25T10:21:40Z

Sean Christopherson <seanjc@google.com> writes:

> When querying whether or not eVMCS is enabled on behalf of the guest,
> treat eVMCS as enable if and only if Hyper-V is enabled/exposed to the
> guest.
>
> Note, flows that come from the host, e.g. KVM_SET_NESTED_STATE, must NOT
> check for Hyper-V being enabled as KVM doesn't require guest CPUID to be
> set before most ioctls().
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/vmx/evmcs.c | 3 +++
> arch/x86/kvm/vmx/nested.c | 8 ++++----
> arch/x86/kvm/vmx/vmx.c | 3 +--
> arch/x86/kvm/vmx/vmx.h | 10 ++++++++++
> 4 files changed, 18 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/evmcs.c b/arch/x86/kvm/vmx/evmcs.c
> index 6a61b1ae7942..9139c70b6008 100644
> --- a/arch/x86/kvm/vmx/evmcs.c
> +++ b/arch/x86/kvm/vmx/evmcs.c
> @@ -334,6 +334,9 @@ uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu)
> * versions: lower 8 bits is the minimal version, higher 8 bits is the
> * maximum supported version. KVM supports versions from 1 to
> * KVM_EVMCS_VERSION.
> + *
> + * Note, do not check the Hyper-V is fully enabled in guest CPUID, this
> + * helper is used to _get_ the vCPU's supported CPUID.
> */
> if (kvm_cpu_cap_get(X86_FEATURE_VMX) &&
> (!vcpu || to_vmx(vcpu)->nested.enlightened_vmcs_enabled))
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ddd4367d4826..28f9d64851b3 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1982,7 +1982,7 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
> bool evmcs_gpa_changed = false;
> u64 evmcs_gpa;
>
> - if (likely(!vmx->nested.enlightened_vmcs_enabled))
> + if (likely(!guest_cpuid_has_evmcs(vcpu)))
> return EVMPTRLD_DISABLED;
>
> if (!nested_enlightened_vmentry(vcpu, &evmcs_gpa)) {
> @@ -2863,7 +2863,7 @@ static int nested_vmx_check_controls(struct kvm_vcpu *vcpu,
> nested_check_vm_entry_controls(vcpu, vmcs12))
> return -EINVAL;
>
> - if (to_vmx(vcpu)->nested.enlightened_vmcs_enabled)
> + if (guest_cpuid_has_evmcs(vcpu))
> return nested_evmcs_check_controls(vmcs12);
>
> return 0;
> @@ -3145,7 +3145,7 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
> * L2 was running), map it here to make sure vmcs12 changes are
> * properly reflected.
> */
> - if (vmx->nested.enlightened_vmcs_enabled &&
> + if (guest_cpuid_has_evmcs(vcpu) &&
> vmx->nested.hv_evmcs_vmptr == EVMPTR_MAP_PENDING) {
> enum nested_evmptrld_status evmptrld_status =
> nested_vmx_handle_enlightened_vmptrld(vcpu, false);
> @@ -5067,7 +5067,7 @@ static int handle_vmclear(struct kvm_vcpu *vcpu)
> * state. It is possible that the area will stay mapped as
> * vmx->nested.hv_evmcs but this shouldn't be a problem.
> */
> - if (likely(!vmx->nested.enlightened_vmcs_enabled ||
> + if (likely(!guest_cpuid_has_evmcs(vcpu) ||
> !nested_enlightened_vmentry(vcpu, &evmcs_gpa))) {
> if (vmptr == vmx->nested.current_vmptr)
> nested_release_vmcs12(vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c9b49a09e6b5..d4ed802947d7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1930,8 +1930,7 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> * sanity checking and refuse to boot. Filter all unsupported
> * features out.
> */
> - if (!msr_info->host_initiated &&
> - vmx->nested.enlightened_vmcs_enabled)
> + if (!msr_info->host_initiated && guest_cpuid_has_evmcs(vcpu))
> nested_evmcs_filter_control_msr(msr_info->index,
> &msr_info->data);
> break;
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 24d58c2ffaa3..35c7e6aef301 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -626,4 +626,14 @@ static inline bool vmx_can_use_ipiv(struct kvm_vcpu *vcpu)
> return lapic_in_kernel(vcpu) && enable_ipiv;
> }
>
> +static inline bool guest_cpuid_has_evmcs(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * eVMCS is exposed to the guest if Hyper-V is enabled in CPUID and
> + * eVMCS has been explicitly enabled by userspace.
> + */
> + return vcpu->arch.hyperv_enabled &&
> + to_vmx(vcpu)->nested.enlightened_vmcs_enabled;

I don't quite like 'guest_cpuid_has_evmcs' name as it makes me think
we're checking if eVMCS was exposed in guest CPUID but in fact we don't
do that. eVMCS can be enabled on a vCPU even if it is not exposed in
CPUID (and we should probably keep that to not mandate setting CPUID
before enabling eVMCS).

What about e.g. vcpu_has_evmcs_enabled() instead?

On a related not, any reason to put this to vmx/vmx.h and not
vmx/evmcs.h?

> +}
> +
> #endif /* __KVM_X86_VMX_H */

--
Vitaly