linux-kernel - Re: [PATCH 2/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPU has been created

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Message-ID: <aG4ph7gNK4o3+04i@intel.com>
Date: Wed, 9 Jul 2025 16:34:15 +0800
From: Chao Gao <chao.gao@...el.com>
To: Kai Huang <kai.huang@...el.com>
CC: <seanjc@...gle.com>, <pbonzini@...hat.com>, <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: Re: [PATCH 2/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPU
 has been created

On Wed, Jul 09, 2025 at 05:38:00PM +1200, Kai Huang wrote:
>Reject the KVM_SET_TSC_KHZ VM ioctl when there's vCPU has already been
>created.
>
>The VM scope KVM_SET_TSC_KHZ ioctl is used to set up the default TSC
>frequency that all subsequent created vCPUs use.  It is only intended to
>be called before any vCPU is created.  Allowing it to be called after
>that only results in confusion but nothing good.
>
>Note this is an ABI change.  But currently in Qemu (the de facto
>userspace VMM) only TDX uses this VM ioctl, and it is only called once
>before creating any vCPU, therefore the risk of breaking userspace is
>pretty low.
>
>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 699ca5e74bba..e5e55d549468 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -7194,6 +7194,10 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
> 		u32 user_tsc_khz;
> 
> 		r = -EINVAL;
>+
>+		if (kvm->created_vcpus)
>+			goto out;
>+

shouldn't kvm->lock be held?

> 		user_tsc_khz = (u32)arg;
> 
> 		if (kvm_caps.has_tsc_control &&
>-- 
>2.50.0
>
>