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

Sean Christopherson posted 1 patch 1 month, 1 week ago 
arch/x86/kvm/x86.c | 40 ++++++++++++++++++++--------------------
1 file changed, 20 insertions(+), 20 deletions(-)
[PATCH] KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR
Posted by Sean Christopherson 1 month, 1 week ago 
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
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 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 ff8812f3a129..dd0b5be1514d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4096,47 +4096,47 @@ 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) {
 			/*
 			 * Pairs with the smp_mb__after_atomic() in
@@ -4149,7 +4149,7 @@ 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;
@@ -4167,7 +4167,7 @@ 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;
@@ -4175,7 +4175,7 @@ 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))
@@ -4476,61 +4476,61 @@ 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;

base-commit: 9448598b22c50c8a5bb77a9103e2d49f134c9578
-- 
2.52.0.351.gbe84eed79e-goog
Re: [PATCH] KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR
Posted by Sean Christopherson 3 weeks, 4 days ago 
On Tue, 30 Dec 2025 12:59:48 -0800, Sean Christopherson wrote:
> 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
> 
> [...]

Applied to kvm-x86 misc, thanks!

[1/1] KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR
      https://github.com/kvm-x86/linux/commit/5bb9ac186512

--
https://github.com/kvm-x86/linux/tree/next
Re: [PATCH] KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR
Posted by Jim Mattson 1 month ago 
On Tue, Dec 30, 2025 at 12:59 PM Sean Christopherson <seanjc@google.com> wrote:
>
> 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
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>

Patchew 2.1-devel-b67e4c2

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