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: <78614df5fad7b1deb291501c9b6db7be81b0a157.camel@intel.com>
Date: Thu, 10 Jul 2025 22:53:07 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "Gao, Chao" <chao.gao@...el.com>
CC: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "bp@...en8.de"
	<bp@...en8.de>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, "nikunj@....com"
	<nikunj@....com>, "thomas.lendacky@....com" <thomas.lendacky@....com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"linux-kernel@...r.kernel.org" <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, 2025-07-09 at 06:55 -0700, Sean Christopherson wrote:
> 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.

My bad.  I'll fixup and send out v2 soon,  together with the doc update.