Re: [PATCH v2 24/28] KVM: nVMX: Use sanitized allowed-1 bits for VMX control MSRs

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jim Mattson <jmattson@google.com> writes:

> On Wed, Jun 29, 2022 at 8:07 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>
>> Using raw host MSR values for setting up nested VMX control MSRs is
>> incorrect as some features need to disabled, e.g. when KVM runs as
>> a nested hypervisor on Hyper-V and uses Enlightened VMCS or when a
>> workaround for IA32_PERF_GLOBAL_CTRL is applied. For non-nested VMX, this
>> is done in setup_vmcs_config() and the result is stored in vmcs_config.
>> Use it for setting up allowed-1 bits in nested VMX MSRs too.
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  arch/x86/kvm/vmx/nested.c | 34 ++++++++++++++++------------------
>>  arch/x86/kvm/vmx/nested.h |  2 +-
>>  arch/x86/kvm/vmx/vmx.c    |  5 ++---
>>  3 files changed, 19 insertions(+), 22 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index 88625965f7b7..e5b19b5e6cab 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -6565,8 +6565,13 @@ static u64 nested_vmx_calc_vmcs_enum_msr(void)
>>   * bit in the high half is on if the corresponding bit in the control field
>>   * may be on. See also vmx_control_verify().
>>   */
>> -void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>> +void nested_vmx_setup_ctls_msrs(struct vmcs_config *vmcs_conf, u32 ept_caps)
>>  {
>> +       struct nested_vmx_msrs *msrs = &vmcs_conf->nested;
>> +
>> +       /* Take the allowed-1 bits from KVM's sanitized VMCS configuration. */
>> +       u32 ignore_high;
>> +
>
> Giving this object a name seems gauche.
>
>>         /*
>>          * Note that as a general rule, the high half of the MSRs (bits in
>>          * the control fields which may be 1) should be initialized by the
>> @@ -6583,11 +6588,11 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>>          */
>>
>>         /* pin-based controls */
>> -       rdmsr(MSR_IA32_VMX_PINBASED_CTLS,
>> -               msrs->pinbased_ctls_low,
>> -               msrs->pinbased_ctls_high);
>> +       rdmsr(MSR_IA32_VMX_PINBASED_CTLS, msrs->pinbased_ctls_low, ignore_high);
>
> Perhaps "(u32){0}" rather than "ignore_high"?
>

While this certainly looks like a cool trick (thanks!), both rdmsr() and
'ignore_high' are gone later in the series. I will, however, adopt the
change, even if just to show off)

>>         msrs->pinbased_ctls_low |=
>>                 PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR;
>
> NYC, but why is this one '|=', and the rest just '='? Does there exist
> a CPU that requires more than PIN_BASED_ALWAYSON_WITHOUT_TRUE_MSR?
>

Looking at the commit which introduced this,

commit eabeaaccfca0ed61b8e00a09b8cfa703c4f11b59
Author: Jan Kiszka <jan.kiszka@siemens.com>
Date:   Wed Mar 13 11:30:50 2013 +0100

    KVM: nVMX: Clean up and fix pin-based execution controls

I don't think '|=' is needed. It is, of course, possible that when KVM is
running nested, required-1 bits are mangled by the underlying hypervisor
but this is a) unlikely b) will only be observed by KVM's L1 (which
means we're talking about 3-level nesting here).

Let's be brave and 'fix' '|=' here.

>> +
>> +       msrs->pinbased_ctls_high = vmcs_conf->pin_based_exec_ctrl;
>>         msrs->pinbased_ctls_high &=
>>                 PIN_BASED_EXT_INTR_MASK |
>>                 PIN_BASED_NMI_EXITING |
>> @@ -6598,12 +6603,10 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>>                 PIN_BASED_VMX_PREEMPTION_TIMER;
>>
>>         /* exit controls */
>> -       rdmsr(MSR_IA32_VMX_EXIT_CTLS,
>> -               msrs->exit_ctls_low,
>> -               msrs->exit_ctls_high);
>>         msrs->exit_ctls_low =
>>                 VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR;
>>
>> +       msrs->exit_ctls_high = vmcs_conf->vmexit_ctrl;
>>         msrs->exit_ctls_high &=
>>  #ifdef CONFIG_X86_64
>>                 VM_EXIT_HOST_ADDR_SPACE_SIZE |
>> @@ -6619,11 +6622,10 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>>         msrs->exit_ctls_low &= ~VM_EXIT_SAVE_DEBUG_CONTROLS;
>>
>>         /* entry controls */
>> -       rdmsr(MSR_IA32_VMX_ENTRY_CTLS,
>> -               msrs->entry_ctls_low,
>> -               msrs->entry_ctls_high);
>>         msrs->entry_ctls_low =
>>                 VM_ENTRY_ALWAYSON_WITHOUT_TRUE_MSR;
>> +
>> +       msrs->entry_ctls_high = vmcs_conf->vmentry_ctrl;
>>         msrs->entry_ctls_high &=
>>  #ifdef CONFIG_X86_64
>>                 VM_ENTRY_IA32E_MODE |
>> @@ -6637,11 +6639,10 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>>         msrs->entry_ctls_low &= ~VM_ENTRY_LOAD_DEBUG_CONTROLS;
>>
>>         /* cpu-based controls */
>> -       rdmsr(MSR_IA32_VMX_PROCBASED_CTLS,
>> -               msrs->procbased_ctls_low,
>> -               msrs->procbased_ctls_high);
>>         msrs->procbased_ctls_low =
>>                 CPU_BASED_ALWAYSON_WITHOUT_TRUE_MSR;
>> +
>> +       msrs->procbased_ctls_high = vmcs_conf->cpu_based_exec_ctrl;
>>         msrs->procbased_ctls_high &=
>>                 CPU_BASED_INTR_WINDOW_EXITING |
>>                 CPU_BASED_NMI_WINDOW_EXITING | CPU_BASED_USE_TSC_OFFSETTING |
>> @@ -6675,12 +6676,9 @@ void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps)
>>          * depend on CPUID bits, they are added later by
>>          * vmx_vcpu_after_set_cpuid.
>>          */
>> -       if (msrs->procbased_ctls_high & CPU_BASED_ACTIVATE_SECONDARY_CONTROLS)
>> -               rdmsr(MSR_IA32_VMX_PROCBASED_CTLS2,
>> -                     msrs->secondary_ctls_low,
>> -                     msrs->secondary_ctls_high);
>> -
>>         msrs->secondary_ctls_low = 0;
>> +
>> +       msrs->secondary_ctls_high = vmcs_conf->cpu_based_2nd_exec_ctrl;
>>         msrs->secondary_ctls_high &=
>>                 SECONDARY_EXEC_DESC |
>>                 SECONDARY_EXEC_ENABLE_RDTSCP |
>> diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
>> index c92cea0b8ccc..fae047c6204b 100644
>> --- a/arch/x86/kvm/vmx/nested.h
>> +++ b/arch/x86/kvm/vmx/nested.h
>> @@ -17,7 +17,7 @@ enum nvmx_vmentry_status {
>>  };
>>
>>  void vmx_leave_nested(struct kvm_vcpu *vcpu);
>> -void nested_vmx_setup_ctls_msrs(struct nested_vmx_msrs *msrs, u32 ept_caps);
>> +void nested_vmx_setup_ctls_msrs(struct vmcs_config *vmcs_conf, u32 ept_caps);
>>  void nested_vmx_hardware_unsetup(void);
>>  __init int nested_vmx_hardware_setup(int (*exit_handlers[])(struct kvm_vcpu *));
>>  void nested_vmx_set_vmcs_shadowing_bitmap(void);
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 5f7ef1f8d2c6..5d4158b7421c 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7310,7 +7310,7 @@ static int __init vmx_check_processor_compat(void)
>>         if (setup_vmcs_config(&vmcs_conf, &vmx_cap) < 0)
>>                 return -EIO;
>>         if (nested)
>> -               nested_vmx_setup_ctls_msrs(&vmcs_conf.nested, vmx_cap.ept);
>> +               nested_vmx_setup_ctls_msrs(&vmcs_conf, vmx_cap.ept);
>>         if (memcmp(&vmcs_config, &vmcs_conf, sizeof(struct vmcs_config)) != 0) {
>>                 printk(KERN_ERR "kvm: CPU %d feature inconsistency!\n",
>>                                 smp_processor_id());
>> @@ -8285,8 +8285,7 @@ static __init int hardware_setup(void)
>>         setup_default_sgx_lepubkeyhash();
>>
>>         if (nested) {
>> -               nested_vmx_setup_ctls_msrs(&vmcs_config.nested,
>> -                                          vmx_capability.ept);
>> +               nested_vmx_setup_ctls_msrs(&vmcs_config, vmx_capability.ept);
>>
>>                 r = nested_vmx_hardware_setup(kvm_vmx_exit_handlers);
>>                 if (r)
>> --
>> 2.35.3
>>
>

-- 
Vitaly