linux-kernel - Re: [PATCH v3 11/25] KVM: VMX: Get rid of eVMCS specific VMX controls sanitization

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Date:   Tue, 12 Jul 2022 14:14:06 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v3 11/25] KVM: VMX: Get rid of eVMCS specific VMX
 controls sanitization

Maxim Levitsky <mlevitsk@...hat.com> writes:

> On Fri, 2022-07-08 at 16:42 +0200, Vitaly Kuznetsov wrote:
>> With the updated eVMCSv1 definition, there's no known 'problematic'
>> controls which are exposed in VMX control MSRs but are not present in
>> eVMCSv1. Get rid of VMX control MSRs filtering for KVM on Hyper-V.
>
> If I understand correctly we are taking about running KVM as a nested guest of Hyper-V here:
>
> Don't we need to check the new CPUID bit and only then use the new fields of eVMCS,
> aka check that the 'cpu' supports the updated eVMCS version?
>

I've checked various Hyper-V versions available around and it seems
there's no need for that: these new features are exposed in VMX control
MSRs only when the updated eVMCS is supported.

We can, in theory, preserve the filtering for non-updated eVMCS verison
but I'd vote for putting a WARN_ON() or something around: we can
eventually get rid of it in case we don't get any reports.

> Best regards,
> 	Maxim Levitsky
>
>
>
>> 
>> Note: VMX control MSRs filtering for Hyper-V on KVM
>> (nested_evmcs_filter_control_msr()) stays as even the updated eVMCSv1
>> definition doesn't have all the features implemented by KVM and some
>> fields are still missing. Moreover, nested_evmcs_filter_control_msr()
>> has to support the original eVMCSv1 version when VMM wishes so.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/vmx/evmcs.c | 13 -------------
>>  arch/x86/kvm/vmx/evmcs.h |  1 -
>>  arch/x86/kvm/vmx/vmx.c   |  5 -----
>>  3 files changed, 19 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx/evmcs.c b/arch/x86/kvm/vmx/evmcs.c
>> index 52a53debd806..b5cfbf7d487b 100644
>> --- a/arch/x86/kvm/vmx/evmcs.c
>> +++ b/arch/x86/kvm/vmx/evmcs.c
>> @@ -320,19 +320,6 @@ const struct evmcs_field vmcs_field_to_evmcs_1[] = {
>>  };
>>  const unsigned int nr_evmcs_1_fields = ARRAY_SIZE(vmcs_field_to_evmcs_1);
>>  
>> -#if IS_ENABLED(CONFIG_HYPERV)
>> -__init void evmcs_sanitize_exec_ctrls(struct vmcs_config *vmcs_conf)
>> -{
>> -       vmcs_conf->cpu_based_exec_ctrl &= ~EVMCS1_UNSUPPORTED_EXEC_CTRL;
>> -       vmcs_conf->pin_based_exec_ctrl &= ~EVMCS1_UNSUPPORTED_PINCTRL;
>> -       vmcs_conf->cpu_based_2nd_exec_ctrl &= ~EVMCS1_UNSUPPORTED_2NDEXEC;
>> -       vmcs_conf->cpu_based_3rd_exec_ctrl = 0;
>> -
>> -       vmcs_conf->vmexit_ctrl &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
>> -       vmcs_conf->vmentry_ctrl &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
>> -}
>> -#endif
>> -
>>  bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 *evmcs_gpa)
>>  {
>>         struct hv_vp_assist_page assist_page;
>> diff --git a/arch/x86/kvm/vmx/evmcs.h b/arch/x86/kvm/vmx/evmcs.h
>> index 4b809c79ae63..0feac101cce4 100644
>> --- a/arch/x86/kvm/vmx/evmcs.h
>> +++ b/arch/x86/kvm/vmx/evmcs.h
>> @@ -203,7 +203,6 @@ static inline void evmcs_load(u64 phys_addr)
>>         vp_ap->enlighten_vmentry = 1;
>>  }
>>  
>> -__init void evmcs_sanitize_exec_ctrls(struct vmcs_config *vmcs_conf);
>>  #else /* !IS_ENABLED(CONFIG_HYPERV) */
>>  static __always_inline void evmcs_write64(unsigned long field, u64 value) {}
>>  static inline void evmcs_write32(unsigned long field, u32 value) {}
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index b4915d841357..dd905ad72637 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -2689,11 +2689,6 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>>         vmcs_conf->vmexit_ctrl         = _vmexit_control;
>>         vmcs_conf->vmentry_ctrl        = _vmentry_control;
>>  
>> -#if IS_ENABLED(CONFIG_HYPERV)
>> -       if (enlightened_vmcs)
>> -               evmcs_sanitize_exec_ctrls(vmcs_conf);
>> -#endif
>> -
>>         return 0;
>>  }
>>  
>
>

-- 
Vitaly