linux-kernel - [PATCH 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest

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Message-ID: <8aebb276ab413f2e4a6512286bd8b6def3596dfe.1752038725.git.kai.huang@intel.com>
Date: Wed,  9 Jul 2025 17:37:59 +1200
From: Kai Huang <kai.huang@...el.com>
To: seanjc@...gle.com,
	pbonzini@...hat.com
Cc: kvm@...r.kernel.org,
	thomas.lendacky@....com,
	nikunj@....com,
	bp@...en8.de,
	isaku.yamahata@...el.com,
	xiaoyao.li@...el.com,
	rick.p.edgecombe@...el.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest

Reject KVM_SET_TSC_KHZ vCPU ioctl if guest's TSC is protected and not
changeable by KVM.

For such TSC protected guests, e.g. TDX guests, typically the TSC is
configured once at VM level before any vCPU are created and remains
unchanged during VM's lifetime.  KVM provides the KVM_SET_TSC_KHZ VM
scope ioctl to allow the userspace VMM to configure the TSC of such VM.
After that the userspace VMM is not supposed to call the KVM_SET_TSC_KHZ
vCPU scope ioctl anymore when creating the vCPU.

The de facto userspace VMM Qemu does this for TDX guests.  The upcoming
SEV-SNP guests with Secure TSC should follow.

Note this could be a break of ABI.  But for now only TDX guests are TSC
protected and only Qemu supports TDX, thus in practice this should not
break any existing userspace.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Kai Huang <kai.huang@...el.com>
---
 arch/x86/kvm/x86.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2806f7104295..699ca5e74bba 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6186,6 +6186,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		u32 user_tsc_khz;

 		r = -EINVAL;
+
+		if (vcpu->arch.guest_tsc_protected)
+			goto out;
+
 		user_tsc_khz = (u32)arg;

 		if (kvm_caps.has_tsc_control &&
-- 
2.50.0