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: <aHTDX1IQuyWYUwL+@intel.com>
Date: Mon, 14 Jul 2025 16:44:15 +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 v2 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when
 vCPUs have been created

On Mon, Jul 14, 2025 at 10:20:19AM +1200, 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>

Reviewed-by: Chao Gao <chao.gao@...el.com>