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

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> On Thu, Jan 16, 2020 at 09:51:47AM +0100, Vitaly Kuznetsov wrote:
>> Liran Alon <liran.alon@xxxxxxxxxx> writes:
>> 
>> >> On 16 Jan 2020, at 1:27, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>> >> 
>> >> On Wed, Jan 15, 2020 at 06:10:13PM +0100, Vitaly Kuznetsov 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.
>> >> 
>> >> As in, Qemu is blindly throwing values at KVM and complains on failure?
>> >> That seems like a Qemu bug, especially since Qemu needs to explicitly do
>> >> KVM_CAP_HYPERV_ENLIGHTENED_VMCS to enable eVMCS.
>> >
>> > See: https://patchwork.kernel.org/patch/11316021/
>> > For more context.
>> 
>> Ya,
>> 
>> while it would certainly be possible to require that userspace takes
>> into account KVM_CAP_HYPERV_ENLIGHTENED_VMCS (which is an opt-in) when
>> doing KVM_SET_MSRS there doesn't seem to be an existing (easy) way to
>> figure out which VMX controls were filtered out after enabling
>> KVM_CAP_HYPERV_ENLIGHTENED_VMCS: KVM_GET_MSRS returns global
>> &vmcs_config.nested values for VMX MSRs (vmx_get_msr_feature()).
>
> Ah, I was looking at the call to vmx_get_vmx_msr(&vmx->nested.msrs, ...)
> in vmx_get_msr().
>
> Why not just do this in Qemu?  IMO that's not a major ask, e.g. Qemu is
> doing a decent amount of manual adjustment anyways.  And Qemu isn't even
> using the result of KVM_GET_MSRS so I don't think it's fair to say this is
> solely KVM's fault.
>
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index 1d10046a6c..6545bb323e 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -2623,6 +2623,23 @@ static void kvm_msr_entry_add_vmx(X86CPU *cpu, FeatureWordArray f)
>               MSR_VMX_EPT_UC | MSR_VMX_EPT_WB : 0);
>      uint64_t fixed_vmx_ept_vpid = kvm_vmx_ept_vpid & fixed_vmx_ept_mask;
>
> +    /* Hyper-V's eVMCS does't support certain features, adjust accordingly. */
> +    if (cpu->hyperv_evmcs) {
> +        f[FEAT_VMX_PINBASED_CTLS] &= ~(VMX_PIN_BASED_VMX_PREEMPTION_TIMER |
> +                                       VMX_PIN_BASED_POSTED_INTR);
> +        f[FEAT_VMX_EXIT_CTLS] &= ~VMX_VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL;
> +        f[FEAT_VMX_ENTRY_CTLS] &= ~VMX_VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
> +        f[FEAT_VMX_SECONDARY_CTLS] &= ~(VMX_SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY |
> +                                        VMX_SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES |
> +                                        VMX_SECONDARY_EXEC_APIC_REGISTER_VIRT |
> +                                        VMX_SECONDARY_EXEC_ENABLE_PML |
> +                                        VMX_SECONDARY_EXEC_ENABLE_VMFUNC |
> +                                        VMX_SECONDARY_EXEC_SHADOW_VMCS |
> +                                        /* VMX_SECONDARY_EXEC_TSC_SCALING | */
> +                                        VMX_SECONDARY_EXEC_PAUSE_LOOP_EXITING);
> +        f[FEAT_VMX_VMFUNC]         &= ~MSR_VMX_VMFUNC_EPT_SWITCHING;
> +    }
> +
>      kvm_msr_entry_add(cpu, MSR_IA32_VMX_TRUE_PROCBASED_CTLS,
>                        make_vmx_msr_value(MSR_IA32_VMX_TRUE_PROCBASED_CTLS,
>                                           f[FEAT_VMX_PROCBASED_CTLS]));
>

I accuse you of not reading my PATCH0 :-)

https://lists.nongnu.org/archive/html/qemu-devel/2020-01/msg00123.html
does exactly this :-) 

P.S. I expect Paolo to comment on which hack he hates less :-)

-- 
Vitaly