linux-kernel - Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable

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Date:   Thu, 16 Jan 2020 08:19:28 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Liran Alon <liran.alon@...cle.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization
 out of nested_enable_evmcs()

On Thu, Jan 16, 2020 at 09:51:47AM +0100, Vitaly Kuznetsov wrote:
> Liran Alon <liran.alon@...cle.com> writes:
> 
> >> On 16 Jan 2020, at 1:27, Sean Christopherson <sean.j.christopherson@...el.com> 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]));