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

Sean Christopherson
2020-01-24T17:25:15Z

On Thu, Jan 23, 2020 at 08:09:03PM +0100, Vitaly Kuznetsov wrote:
> Vitaly Kuznetsov <vkuznets@redhat.com> writes:
>
> > Paolo Bonzini <pbonzini@redhat.com> writes:
> >
> >> On 22/01/20 17:29, Vitaly Kuznetsov wrote:
> >>> Yes, in case we're back to the idea to filter things out in QEMU we can
> >>> do this. What I don't like is that every other userspace which decides
> >>> to enable eVMCS will have to perform the exact same surgery as in case
> >>> it sets allow_unsupported_controls=0 it'll have to know (hardcode) the
> >>> filtering (or KVM_SET_MSRS will fail) and in case it opts for
> >>> allow_unsupported_controls=1 Windows guests just won't boot without the
> >>> filtering.
> >>>
> >>> It seems to be 1:1, eVMCSv1 requires the filter.
> >>
> >> Yes, that's the point. It *is* a hack in KVM, but it is generally
> >> preferrable to have an easier API for userspace, if there's only one way
> >> to do it.
> >>
> >> Though we could be a bit more "surgical" and only remove
> >> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES---thus minimizing the impact on
> >> non-eVMCS guests. Vitaly, can you prepare a v2 that does that and adds
> >> a huge "hack alert" comment that explains the discussion?
> >
> > Yes, sure. I'd like to do more testing to make sure filtering out
> > SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES is enough for other Hyper-V
> > versions too (who knows how many bugs are there :-)
>
> ... and the answer is -- more than one :-)
>
> I've tested Hyper-V 2016/2019 BIOS and UEFI-booted and the minimal
> viable set seems to be:
>
> MSR_IA32_VMX_PROCBASED_CTLS2:
> ~SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES
>
> MSR_IA32_VMX_ENTRY_CTLS/MSR_IA32_VMX_TRUE_ENTRY_CTLS:
> ~VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL
>
> MSR_IA32_VMX_EXIT_CTLS/MSR_IA32_VMX_TRUE_EXIT_CTLS:
> ~VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL
>
> with these filtered out all 4 versions are at least able to boot with >1
> vCPU and run a nested guest (different from Windows management
> partition).
>
> This still feels a bit fragile as who knows under which circumstances
> Hyper-V might want to enable additional (missing) controls.

No strong opinion, I'm good either way.

> If there are no objections and if we still think it would be beneficial
> to minimize the list of controls we filter out (and not go with the full
> set like my RFC suggests), I'll prepare v2. (v1, actually, this was RFC).

One last idea, can we keep the MSR filtering as is and add the hack in
vmx_restore_control_msr()? That way the (userspace) host and guest see
the same values when reading the affected MSRs, and eVMCS wouldn't need
it's own hook to do consistency checks.

@@ -1181,28 +1181,38 @@ static int
vmx_restore_control_msr(struct vcpu_vmx *vmx, u32 msr_index, u64 data)
{
u64 supported;
- u32 *lowp, *highp;
+ u32 *lowp, *highp, evmcs_unsupported;

switch (msr_index) {
case MSR_IA32_VMX_TRUE_PINBASED_CTLS:
lowp = &vmx->nested.msrs.pinbased_ctls_low;
highp = &vmx->nested.msrs.pinbased_ctls_high;
+ if (vmx->nested.enlightened_vmcs_enabled)
+ evmcs_unsupported = EVMCS1_UNSUPPORTED_PINCTRL;
break;
case MSR_IA32_VMX_TRUE_PROCBASED_CTLS:
lowp = &vmx->nested.msrs.procbased_ctls_low;
highp = &vmx->nested.msrs.procbased_ctls_high;
+ if (vmx->nested.enlightened_vmcs_enabled)
+ evmcs_unsupported = 0;
break;
case MSR_IA32_VMX_TRUE_EXIT_CTLS:
lowp = &vmx->nested.msrs.exit_ctls_low;
highp = &vmx->nested.msrs.exit_ctls_high;
+ if (vmx->nested.enlightened_vmcs_enabled)
+ evmcs_unsupported = EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
break;
case MSR_IA32_VMX_TRUE_ENTRY_CTLS:
lowp = &vmx->nested.msrs.entry_ctls_low;
highp = &vmx->nested.msrs.entry_ctls_high;
+ if (vmx->nested.enlightened_vmcs_enabled)
+ evmcs_unsupported = EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
break;
case MSR_IA32_VMX_PROCBASED_CTLS2:
lowp = &vmx->nested.msrs.secondary_ctls_low;
highp = &vmx->nested.msrs.secondary_ctls_high;
+ if (vmx->nested.enlightened_vmcs_enabled)
+ evmcs_unsupported = EVMCS1_UNSUPPORTED_2NDEXEC;
break;
default:
BUG();
@@ -1210,6 +1220,9 @@ vmx_restore_control_msr(struct vcpu_vmx *vmx, u32 msr_index, u64 data)

supported = vmx_control_msr(*lowp, *highp);

+ /* HACK! */
+ data &= ~(u64)evmcs_unsupported << 32;
+
/* Check must-be-1 bits are still 1. */
if (!is_bitwise_subset(data, supported, GENMASK_ULL(31, 0)))