[PATCH 1/4] KVM: x86: hyper-v: Limit guest to writing zero to HV_X64_MSR_TSC_EMULATION_STATUS

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Marcelo Tosatti <mtosatti@redhat.com>
Subject: [PATCH 1/4] KVM: x86: hyper-v: Limit guest to writing zero to HV_X64_MSR_TSC_EMULATION_STATUS
Date: Mon, 15 Mar 2021 15:37:03 +0100	[thread overview]
Message-ID: <20210315143706.859293-2-vkuznets@redhat.com> (raw)
In-Reply-To: <20210315143706.859293-1-vkuznets@redhat.com>

HV_X64_MSR_TSC_EMULATION_STATUS indicates whether TSC accesses are emulated
after migration (to accommodate for a different host TSC frequency when TSC
scaling is not supported; we don't implement this in KVM). Guest can use
the same MSR to stop TSC access emulation by writing zero. Writing anything
else is forbidden.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kvm/hyperv.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 58fa8c029867..eefb85b86fe8 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1229,6 +1229,9 @@ static int kvm_hv_set_msr_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data,
 		hv->hv_tsc_emulation_control = data;
 		break;
 	case HV_X64_MSR_TSC_EMULATION_STATUS:
+		if (data && !host)
+			return 1;
+
 		hv->hv_tsc_emulation_status = data;
 		break;
 	case HV_X64_MSR_TIME_REF_COUNT:
-- 
2.30.2

