Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable_evmcs()

Liran Alon
2020-01-15T22:49:33Z

> On 15 Jan 2020, at 19:10, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> With fine grained VMX feature enablement QEMU>=4.2 tries to do KVM_SET_MSRS
> with default (matching CPU model) values and in case eVMCS is also enabled,
> fails.
>
> It would be possible to drop VMX feature filtering completely and make
> this a guest's responsibility: if it decides to use eVMCS it should know
> which fields are available and which are not. Hyper-V mostly complies to
> this, however, there is at least one problematic control:
> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES
> which Hyper-V enables. As there is no 'apic_addr_field' in eVMCS, we
> fail to handle this properly in KVM. It is unclear how this is supposed
> to work, genuine Hyper-V doesn't expose the control so it is possible that
> this is just a bug (in Hyper-V).

Have you tried contacted someone at Hyper-V team about this?

>
> Move VMX controls sanitization from nested_enable_evmcs() to vmx_get_msr(),
> this allows userspace to keep setting controls it wants and at the same
> time hides them from the guest.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/vmx/evmcs.c | 38 ++++++++++++++++++++++++++++++++------
> arch/x86/kvm/vmx/evmcs.h | 1 +
> arch/x86/kvm/vmx/vmx.c | 10 ++++++++--
> 3 files changed, 41 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/evmcs.c b/arch/x86/kvm/vmx/evmcs.c
> index 89c3e0caf39f..b5d6582ba589 100644
> --- a/arch/x86/kvm/vmx/evmcs.c
> +++ b/arch/x86/kvm/vmx/evmcs.c
> @@ -346,6 +346,38 @@ uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu)
> return 0;
> }
>
> +void nested_evmcs_filter_control_msr(u32 msr_index, u64 *pdata)
> +{
> + u32 ctl_low = (u32)*pdata, ctl_high = (u32)(*pdata >> 32);

Nit: I dislike defining & initialising multiple local vars on same line.

> + /*
> + * Enlightened VMCS doesn't have certain fields, make sure we don't
> + * expose unsupported controls to L1.
> + */
> +
> + switch (msr_index) {
> + case MSR_IA32_VMX_PINBASED_CTLS:
> + case MSR_IA32_VMX_TRUE_PINBASED_CTLS:
> + ctl_high &= ~EVMCS1_UNSUPPORTED_PINCTRL;
> + break;
> + case MSR_IA32_VMX_EXIT_CTLS:
> + case MSR_IA32_VMX_TRUE_EXIT_CTLS:
> + ctl_high &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
> + break;
> + case MSR_IA32_VMX_ENTRY_CTLS:
> + case MSR_IA32_VMX_TRUE_ENTRY_CTLS:
> + ctl_high &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
> + break;
> + case MSR_IA32_VMX_PROCBASED_CTLS2:
> + ctl_high &= ~EVMCS1_UNSUPPORTED_2NDEXEC;
> + break;
> + case MSR_IA32_VMX_VMFUNC:
> + ctl_low &= ~EVMCS1_UNSUPPORTED_VMFUNC;
> + break;
> + }
> +
> + *pdata = ctl_low | ((u64)ctl_high << 32);
> +}
> +
> int nested_enable_evmcs(struct kvm_vcpu *vcpu,
> uint16_t *vmcs_version)
> {
> @@ -356,11 +388,5 @@ int nested_enable_evmcs(struct kvm_vcpu *vcpu,
> if (vmcs_version)
> *vmcs_version = nested_get_evmcs_version(vcpu);
>
> - vmx->nested.msrs.pinbased_ctls_high &= ~EVMCS1_UNSUPPORTED_PINCTRL;
> - vmx->nested.msrs.entry_ctls_high &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
> - vmx->nested.msrs.exit_ctls_high &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
> - vmx->nested.msrs.secondary_ctls_high &= ~EVMCS1_UNSUPPORTED_2NDEXEC;
> - vmx->nested.msrs.vmfunc_controls &= ~EVMCS1_UNSUPPORTED_VMFUNC;
> -
> return 0;
> }
> diff --git a/arch/x86/kvm/vmx/evmcs.h b/arch/x86/kvm/vmx/evmcs.h
> index 07ebf6882a45..b88d9807a796 100644
> --- a/arch/x86/kvm/vmx/evmcs.h
> +++ b/arch/x86/kvm/vmx/evmcs.h
> @@ -201,5 +201,6 @@ bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 *evmcs_gpa);
> uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu);
> int nested_enable_evmcs(struct kvm_vcpu *vcpu,
> uint16_t *vmcs_version);
> +void nested_evmcs_filter_control_msr(u32 msr_index, u64 *pdata);
>
> #endif /* __KVM_X86_VMX_EVMCS_H */
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e3394c839dea..8eb74618b8d8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1849,8 +1849,14 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_IA32_VMX_BASIC ... MSR_IA32_VMX_VMFUNC:
> if (!nested_vmx_allowed(vcpu))
> return 1;
> - return vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
> - &msr_info->data);
> + if (vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
> + &msr_info->data))
> + return 1;
> + if (!msr_info->host_initiated &&
> + vmx->nested.enlightened_vmcs_enabled)
> + nested_evmcs_filter_control_msr(msr_info->index,
> + &msr_info->data);
> + break;

Nit: It seems more elegant to me to put the call to nested_evmcs_filter_control_msr() inside vmx_get_vmx_msr().

The patch itself makes sense to me and looks correct.
Reviewed-by: Liran Alon <liran.alon@oracle.com>

-Liran