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SubjectRe: [PATCH 0/3] KVM: x86: SVM INVPCID fix, and cleanups
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On 12/02/21 01:34, Sean Christopherson wrote:
> Fix an INVPCID bug on SVM where it fails to injected a #UD when INVPCID is
> supported but not exposed to the guest. Do a bit of cleanup in patch 02
> now that both VMX and SVM support PCID/INVPCID.
>
> Patch 03 address KVM behavior that has long confused the heck out of me.
> KVM currently allows enabling INVPCID if and only if PCID is also enabled
> for the guest, the justification being that the guest will see incorrect
> fault behavior (#UD instead of #GP) due to the way the VMCS control works.
>
> But that makes no sense, because nothing is forcing KVM to disable INVCPID
> in the VMCS when PCID is disabled. AFACIT, the myth was the result of a
> bug in the original _submission_, not even the original _commit_ was buggy.
>
> Digging back, the very original submission had this code, where
> vmx_pcid_supported() was further conditioned on EPT being enabled. This
> would lead to the buggy scenario of unexpected #UD, as a host with PCID
> and INVCPID would fail to enable INVPCID if EPT was disabled.
>
>>> + if (vmx_pcid_supported()) {
>>> + exec_control = vmcs_read32(SECONDARY_VM_EXEC_CONTROL);
>>> + if (exec_control & SECONDARY_EXEC_ENABLE_INVPCID) {
>>> + best = kvm_find_cpuid_entry(vcpu, 0x1, 0);
>>> + if (best && (best->ecx & bit(X86_FEATURE_PCID)))
>>> + vmx->invpcid_enabled = true;
>>> + else {
>>> + exec_control &= ~SECONDARY_EXEC_ENABLE_INVPCID;
>>> + vmcs_write32(SECONDARY_VM_EXEC_CONTROL,
>>> + exec_control);
>>> + best = kvm_find_cpuid_entry(vcpu, 0x7, 0);
>>> + best->ecx &= ~bit(X86_FEATURE_INVPCID);
>>> + }
>>> + }
>>> + }
>
> The incorrect behavior is especially problematic now that SVM also
> supports INVCPID, as KVM allows !PCID && INVPCID on SVM but not on VMX.
>
> Patches to fix kvm-unit-tests are also incoming...
>
> Sean Christopherson (3):
> KVM: SVM: Intercept INVPCID when it's disabled to inject #UD
> KVM: x86: Advertise INVPCID by default
> KVM: VMX: Allow INVPCID in guest without PCID
>
> arch/x86/kvm/cpuid.c | 2 +-
> arch/x86/kvm/svm/svm.c | 11 ++++-------
> arch/x86/kvm/vmx/vmx.c | 14 ++------------
> 3 files changed, 7 insertions(+), 20 deletions(-)
>

Queued, thanks.

Paolo

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