[PATCH 2/2] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS

[Maxim Levitsky <mlevitsk@redhat.com>]

This quirk reflects the fact that we currently treat MSR_IA32_TSC
and MSR_TSC_ADJUST access by the host (e.g qemu) in a way that is different
compared to an access from the guest.

For host's MSR_IA32_TSC read we currently always return L1 TSC value, and for
host's write we do the tsc synchronization.

For host's MSR_TSC_ADJUST write, we don't make the tsc 'jump' as we should
for this msr.

When the hypervisor uses the new TSC GET/SET state ioctls, all of this is no
longer needed, thus leave this enabled only with a quirk
which the hypervisor can disable.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/include/uapi/asm/kvm.h |  1 +
 arch/x86/kvm/x86.c              | 19 ++++++++++++++-----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 8e76d3701db3f..2a60fc6674164 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -404,6 +404,7 @@ struct kvm_sync_regs {
 #define KVM_X86_QUIRK_LAPIC_MMIO_HOLE	   (1 << 2)
 #define KVM_X86_QUIRK_OUT_7E_INC_RIP	   (1 << 3)
 #define KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT (1 << 4)
+#define KVM_X86_QUIRK_TSC_HOST_ACCESS      (1 << 5)
 
 #define KVM_STATE_NESTED_FORMAT_VMX	0
 #define KVM_STATE_NESTED_FORMAT_SVM	1
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4f0ae9cb14b8a..46a2111d54840 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3091,7 +3091,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_IA32_TSC_ADJUST:
 		if (guest_cpuid_has(vcpu, X86_FEATURE_TSC_ADJUST)) {
-			if (!msr_info->host_initiated) {
+			if (!msr_info->host_initiated ||
+			    !kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_TSC_HOST_ACCESS)) {
 				s64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
 				adjust_tsc_offset_guest(vcpu, adj);
 			}
@@ -3118,7 +3119,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vcpu->arch.msr_ia32_power_ctl = data;
 		break;
 	case MSR_IA32_TSC:
-		if (msr_info->host_initiated) {
+		if (msr_info->host_initiated &&
+		    kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_TSC_HOST_ACCESS)) {
 			kvm_synchronize_tsc(vcpu, data);
 		} else {
 			u64 adj = kvm_compute_tsc_offset(vcpu, data) - vcpu->arch.l1_tsc_offset;
@@ -3409,17 +3411,24 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = vcpu->arch.msr_ia32_power_ctl;
 		break;
 	case MSR_IA32_TSC: {
+		u64 tsc_offset;
+
 		/*
 		 * Intel SDM states that MSR_IA32_TSC read adds the TSC offset
 		 * even when not intercepted. AMD manual doesn't explicitly
 		 * state this but appears to behave the same.
 		 *
-		 * On userspace reads and writes, however, we unconditionally
+		 * On userspace reads and writes, when KVM_X86_QUIRK_SPECIAL_TSC_READ
+		 * is present, however, we unconditionally
 		 * return L1's TSC value to ensure backwards-compatible
 		 * behavior for migration.
 		 */
-		u64 tsc_offset = msr_info->host_initiated ? vcpu->arch.l1_tsc_offset :
-							    vcpu->arch.tsc_offset;
+
+		if (msr_info->host_initiated &&
+		    kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_TSC_HOST_ACCESS))
+			tsc_offset = vcpu->arch.l1_tsc_offset;
+		else
+			tsc_offset = vcpu->arch.tsc_offset;
 
 		msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) + tsc_offset;
 		break;
-- 
2.26.2