[PATCH 6.18 497/752] KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR

Sasha Levin
2026-02-28T17:55:12Z

From: Sean Christopherson <seanjc@google.com>

[ Upstream commit 5bb9ac1865123356337a389af935d3913ee917ed ]

Return KVM_MSR_RET_UNSUPPORTED instead of '1' (which for all intents and
purposes means "invalid") when rejecting accesses to KVM PV MSRs to adhere
to KVM's ABI of allowing host reads and writes of '0' to MSRs that are
advertised to userspace via KVM_GET_MSR_INDEX_LIST, even if the vCPU model
doesn't support the MSR.

E.g. running a QEMU VM with

-cpu host,-kvmclock,kvm-pv-enforce-cpuid

yields:

qemu: error: failed to set MSR 0x12 to 0x0
qemu: target/i386/kvm/kvm.c:3301: kvm_buf_set_msrs:
Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.

Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
Cc: stable@vger.kernel.org
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20251230205948.4094097-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kvm/x86.c | 40 ++++++++++++++++++++--------------------
1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c075ee23aeade..79d0abaf71dde 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4136,47 +4136,47 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
case MSR_KVM_WALL_CLOCK_NEW:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE2))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

vcpu->kvm->arch.wall_clock = data;
kvm_write_wall_clock(vcpu->kvm, data, 0);
break;
case MSR_KVM_WALL_CLOCK:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

vcpu->kvm->arch.wall_clock = data;
kvm_write_wall_clock(vcpu->kvm, data, 0);
break;
case MSR_KVM_SYSTEM_TIME_NEW:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE2))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

kvm_write_system_time(vcpu, data, false, msr_info->host_initiated);
break;
case MSR_KVM_SYSTEM_TIME:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

kvm_write_system_time(vcpu, data, true, msr_info->host_initiated);
break;
case MSR_KVM_ASYNC_PF_EN:
if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

if (kvm_pv_enable_async_pf(vcpu, data))
return 1;
break;
case MSR_KVM_ASYNC_PF_INT:
if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

if (kvm_pv_enable_async_pf_int(vcpu, data))
return 1;
break;
case MSR_KVM_ASYNC_PF_ACK:
if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;
if (data & 0x1) {
vcpu->arch.apf.pageready_pending = false;
kvm_check_async_pf_completion(vcpu);
@@ -4184,7 +4184,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
case MSR_KVM_STEAL_TIME:
if (!guest_pv_has(vcpu, KVM_FEATURE_STEAL_TIME))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

if (unlikely(!sched_info_on()))
return 1;
@@ -4202,7 +4202,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
case MSR_KVM_PV_EOI_EN:
if (!guest_pv_has(vcpu, KVM_FEATURE_PV_EOI))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

if (kvm_lapic_set_pv_eoi(vcpu, data, sizeof(u8)))
return 1;
@@ -4210,7 +4210,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)

case MSR_KVM_POLL_CONTROL:
if (!guest_pv_has(vcpu, KVM_FEATURE_POLL_CONTROL))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

/* only enable bit supported */
if (data & (-1ULL << 1))
@@ -4511,61 +4511,61 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
break;
case MSR_KVM_WALL_CLOCK:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->kvm->arch.wall_clock;
break;
case MSR_KVM_WALL_CLOCK_NEW:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE2))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->kvm->arch.wall_clock;
break;
case MSR_KVM_SYSTEM_TIME:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.time;
break;
case MSR_KVM_SYSTEM_TIME_NEW:
if (!guest_pv_has(vcpu, KVM_FEATURE_CLOCKSOURCE2))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.time;
break;
case MSR_KVM_ASYNC_PF_EN:
if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.apf.msr_en_val;
break;
case MSR_KVM_ASYNC_PF_INT:
if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.apf.msr_int_val;
break;
case MSR_KVM_ASYNC_PF_ACK:
if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = 0;
break;
case MSR_KVM_STEAL_TIME:
if (!guest_pv_has(vcpu, KVM_FEATURE_STEAL_TIME))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.st.msr_val;
break;
case MSR_KVM_PV_EOI_EN:
if (!guest_pv_has(vcpu, KVM_FEATURE_PV_EOI))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.pv_eoi.msr_val;
break;
case MSR_KVM_POLL_CONTROL:
if (!guest_pv_has(vcpu, KVM_FEATURE_POLL_CONTROL))
- return 1;
+ return KVM_MSR_RET_UNSUPPORTED;

msr_info->data = vcpu->arch.msr_kvm_poll_control;
break;
--
2.51.0