KVM: x86: Emulate IA32_TSC_ADJUST MSR

CPUID.7.0.EBX[1]=1 indicates IA32_TSC_ADJUST MSR 0x3b is supported

Basic design is to emulate the MSR by allowing reads and writes to a guest
vcpu specific location to store the value of the emulated MSR while adding
the value to the vmcs tsc_offset. In this way the IA32_TSC_ADJUST value will
be included in all reads to the TSC MSR whether through rdmsr or rdtsc. This
is of course as long as the "use TSC counter offsetting" VM-execution control
is enabled as well as the IA32_TSC_ADJUST control.

However, because hardware will only return the TSC + IA32_TSC_ADJUST +
vmsc tsc_offset for a guest process when it does and rdtsc (with the correct
settings) the value of our virtualized IA32_TSC_ADJUST must be stored in one
of these three locations. The argument against storing it in the actual MSR
is performance. This is likely to be seldom used while the save/restore is
required on every transition. IA32_TSC_ADJUST was created as a way to solve
some issues with writing TSC itself so that is not an option either.

The remaining option, defined above as our solution has the problem of
returning incorrect vmcs tsc_offset values (unless we intercept and fix, not
done here) as mentioned above. However, more problematic is that storing the
data in vmcs tsc_offset will have a different semantic effect on the system
than does using the actual MSR. This is illustrated in the following example:

The hypervisor set the IA32_TSC_ADJUST, then the guest sets it and a guest
process performs a rdtsc. In this case the guest process will get
TSC + IA32_TSC_ADJUST_hyperviser + vmsc tsc_offset including
IA32_TSC_ADJUST_guest. While the total system semantics changed the semantics
as seen by the guest do not and hence this will not cause a problem.

Signed-off-by: Will Auld <will.auld@intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 95f66136f2d5..b0b8abe688ce 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -831,6 +831,7 @@ static u32 msrs_to_save[] = {
 static unsigned num_msrs_to_save;
 
 static const u32 emulated_msrs[] = {
+	MSR_IA32_TSC_ADJUST,
 	MSR_IA32_TSCDEADLINE,
 	MSR_IA32_MISC_ENABLE,
 	MSR_IA32_MCG_STATUS,
@@ -1135,6 +1136,12 @@ void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
 #endif
 }
 
+static void update_ia32_tsc_adjust_msr(struct kvm_vcpu *vcpu, s64 offset)
+{
+	u64 curr_offset = kvm_x86_ops->read_tsc_offset(vcpu);
+	vcpu->arch.ia32_tsc_adjust_msr += offset - curr_offset;
+}
+
 void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 {
 	struct kvm *kvm = vcpu->kvm;
@@ -1222,6 +1229,8 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
 	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
 
+	if (guest_cpuid_has_tsc_adjust(vcpu) && !msr->host_initiated)
+		update_ia32_tsc_adjust_msr(vcpu, offset);
 	kvm_x86_ops->write_tsc_offset(vcpu, offset);
 	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
 
@@ -1918,6 +1927,15 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_IA32_TSCDEADLINE:
 		kvm_set_lapic_tscdeadline_msr(vcpu, data);
 		break;
+	case MSR_IA32_TSC_ADJUST:
+		if (guest_cpuid_has_tsc_adjust(vcpu)) {
+			if (!msr_info->host_initiated) {
+				u64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
+				kvm_x86_ops->adjust_tsc_offset(vcpu, adj, true);
+			}
+			vcpu->arch.ia32_tsc_adjust_msr = data;
+		}
+		break;
 	case MSR_IA32_MISC_ENABLE:
 		vcpu->arch.ia32_misc_enable_msr = data;
 		break;
@@ -2277,6 +2295,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
 	case MSR_IA32_TSCDEADLINE:
 		data = kvm_get_lapic_tscdeadline_msr(vcpu);
 		break;
+	case MSR_IA32_TSC_ADJUST:
+		data = (u64)vcpu->arch.ia32_tsc_adjust_msr;
+		break;
 	case MSR_IA32_MISC_ENABLE:
 		data = vcpu->arch.ia32_misc_enable_msr;
 		break;
@@ -6607,6 +6628,7 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 	if (!zalloc_cpumask_var(&vcpu->arch.wbinvd_dirty_mask, GFP_KERNEL))
 		goto fail_free_mce_banks;
 
+	vcpu->arch.ia32_tsc_adjust_msr = 0x0;
 	kvm_async_pf_hash_reset(vcpu);
 	kvm_pmu_init(vcpu);