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FAILED: Patch "KVM: x86: Return "unsupported" instead of "invalid" on access to unsupported PV MSR" failed to apply to 6.6-stable tree

PATCH Sasha Levin 1 participant 1 message Mar 1

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Stable branch x86 KVM KVM x86
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Sasha Levin

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The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 5bb9ac1865123356337a389af935d3913ee917ed Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Tue, 30 Dec 2025 12:59:48 -0800
Subject: [PATCH] KVM: x86: Return "unsupported" instead of "invalid" on access
to unsupported PV MSR

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 [email protected]
Reviewed-by Jim Mattson <[email protected]>
Link https://patch.msgid.link/[email protected] open here
Signed-off-by Sean Christopherson <[email protected]>
---
 arch/x86/kvm/x86.c | 40 ++++++++++++++++++++--------------------
 1 file changed, 20 insertions(+), 20 deletions(-)
arch/x86/kvm/x86.c +20 -20
index 211d8c24a4b11..e4418409b468d 100644
@@ -4097,47 +4097,47 @@ 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

@@ -4150,7 +4150,7 @@ 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;

@@ -4168,7 +4168,7 @@ 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;

@@ -4176,7 +4176,7 @@ 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))

@@ -4477,61 +4477,61 @@ 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;
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