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

[Vitaly Kuznetsov]

Liran Alon <liran.alon@xxxxxxxxxx> writes:

>> On 15 Jan 2020, at 19:10, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> 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?
>

Yes, I have.

>> 
>> 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@xxxxxxxxxx>
>> ---
>> 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().
>

Sure, will move it there (in case we actually decide to merge this)

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

Thanks!

-- 
Vitaly