[PATCH v5 08/15] KVM: nVMX: Keep KVM updates to PERF_GLOBAL_CTRL ctrl bits across MSR write

Sean Christopherson
2022-06-08T01:47:10Z

From: Oliver Upton <oupton@google.com>

Since commit 03a8871add95 ("KVM: nVMX: Expose load IA32_PERF_GLOBAL_CTRL
VM-{Entry,Exit} control"), KVM has taken ownership of the "load
IA32_PERF_GLOBAL_CTRL" VMX entry/exit control bits. The ABI is that
these bits will be set in the IA32_VMX_TRUE_{ENTRY,EXIT}_CTLS MSRs if
the guest's CPUID exposes a vPMU that supports the IA32_PERF_GLOBAL_CTRL
MSR (CPUID.0AH:EAX[7:0] > 1), and clear otherwise.

However, commit aedbaf4f6afd ("KVM: x86: Extract kvm_update_cpuid_runtime()
from kvm_update_cpuid()") partially broke KVM ownership of the
aforementioned bits. Before, kvm_update_cpuid() was exercised frequently
when running a guest and constantly applied its own changes to the "load
IA32_PERF_GLOBAL_CTRL" bits. Now, the "load IA32_PERF_GLOBAL_CTRL" bits
are only ever updated after a KVM_SET_CPUID/KVM_SET_CPUID2 ioctl, meaning
that a subsequent MSR write from userspace will clobber these values.

Note that older kernels without commit c44d9b34701d ("KVM: x86: Invoke
vendor's vcpu_after_set_cpuid() after all common updates") still require
that the entry/exit controls be updated from kvm_pmu_refresh(). Leave
the benign call in place to allow for cleaner backporting and punt the
cleanup to a later change.

Uphold the old ABI by reapplying KVM's tweaks to the "load
IA32_PERF_GLOBAL_CTRL" bits after an MSR write from userspace.

Note, the old ABI that is being preserved is misguided KVM behavior that
was introduced by commit 03a8871add95 ("KVM: nVMX: Expose load
IA32_PERF_GLOBAL_CTRL VM-{Entry,Exit} control"). KVM's bogus tweaking of
VMX MSRs was first implemented by commit 5f76f6f5ff96 ("KVM: nVMX: Do not
expose MPX VMX controls when guest MPX disabled") to hack around a QEMU
bug, and that bad behavior was unfortunately applied to PERF_GLOBAL_CTRL
before it could be stamped out.

Fixes: aedbaf4f6afd ("KVM: x86: Extract kvm_update_cpuid_runtime() from kvm_update_cpuid()")
Reported-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Oliver Upton <oupton@google.com>
[sean: explicitly document the original KVM hack, set bits iff CPU
supports the control]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/vmx.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3f1671d7cbe4..73ec4746a4e6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7413,6 +7413,20 @@ void nested_vmx_entry_exit_ctls_update(struct kvm_vcpu *vcpu)
vmx->nested.msrs.exit_ctls_high &= ~VM_EXIT_CLEAR_BNDCFGS;
}
}
+
+ /*
+ * KVM supports a 1-setting of the "load IA32_PERF_GLOBAL_CTRL"
+ * VM-{Entry,Exit} controls if the vPMU supports IA32_PERF_GLOBAL_CTRL.
+ */
+ if (cpu_has_load_perf_global_ctrl()) {
+ if (intel_pmu_has_perf_global_ctrl(vcpu_to_pmu(vcpu))) {
+ vmx->nested.msrs.entry_ctls_high |= VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
+ vmx->nested.msrs.exit_ctls_high |= VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL;
+ } else {
+ vmx->nested.msrs.entry_ctls_high &= ~VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
+ vmx->nested.msrs.exit_ctls_high &= ~VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL;
+ }
+ }
}

static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
--
2.36.1.255.ge46751e96f-goog