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: <aG501qKTDjmcLEyV@google.com>
Date: Wed, 9 Jul 2025 06:55:35 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Kai Huang <kai.huang@...el.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, Chao Gao wrote:
> 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?

Yep.