linux-kernel - Re: [PATCH v2 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created

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Message-ID: <7ea7a720-1271-4021-b6e1-89b9aa3b69ba@amd.com>
Date: Mon, 14 Jul 2025 11:13:10 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Kai Huang <kai.huang@...el.com>, seanjc@...gle.com, pbonzini@...hat.com
Cc: kvm@...r.kernel.org, thomas.lendacky@....com, bp@...en8.de,
 isaku.yamahata@...el.com, xiaoyao.li@...el.com, rick.p.edgecombe@...el.com,
 chao.gao@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when
 vCPUs have been created



On 7/14/2025 3:50 AM, Kai Huang wrote:
> Reject the KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created and
> update the documentation to reflect it.
> 
> The VM scope KVM_SET_TSC_KHZ ioctl is used to set up the default TSC
> frequency that all subsequently created vCPUs can 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>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

LTGM:

Reviewed-by: Nikunj A Dadhania <nikunj@....com>

> ---
>  Documentation/virt/kvm/api.rst | 2 +-
>  arch/x86/kvm/x86.c             | 9 ++++++---
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 43ed57e048a8..e343430ccb01 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -2006,7 +2006,7 @@ frequency is KHz.
>  
>  If the KVM_CAP_VM_TSC_CONTROL capability is advertised, this can also
>  be used as a vm ioctl to set the initial tsc frequency of subsequently
> -created vCPUs.
> +created vCPUs. The vm ioctl must be called before any vCPU is created.
>  
>  4.56 KVM_GET_TSC_KHZ
>  --------------------
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2806f7104295..4051c0cacb92 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7199,9 +7199,12 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>  		if (user_tsc_khz == 0)
>  			user_tsc_khz = tsc_khz;
>  
> -		WRITE_ONCE(kvm->arch.default_tsc_khz, user_tsc_khz);
> -		r = 0;
> -
> +		mutex_lock(&kvm->lock);
> +		if (!kvm->created_vcpus) {
> +			WRITE_ONCE(kvm->arch.default_tsc_khz, user_tsc_khz);
> +			r = 0;
> +		}
> +		mutex_unlock(&kvm->lock);
>  		goto out;
>  	}
>  	case KVM_GET_TSC_KHZ: {