linux-kernel - Re: [PATCH 1/2] KVM: x86: Reject KVM_SET_TSC

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Message-ID: <9b2e872b1948df129e5f893c2fbd9b11d0920696.camel@intel.com>
Date: Thu, 10 Jul 2025 22:52:49 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "nikunj@....com" <nikunj@....com>
CC: "thomas.lendacky@....com" <thomas.lendacky@....com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Edgecombe,
 Rick P" <rick.p.edgecombe@...el.com>, "bp@...en8.de" <bp@...en8.de>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC
 protected guest

On Wed, 2025-07-09 at 14:09 +0530, Nikunj A. Dadhania wrote:
> 
> On 7/9/2025 11:07 AM, Kai Huang wrote:
> > Reject KVM_SET_TSC_KHZ vCPU ioctl if guest's TSC is protected and not
> > changeable by KVM.
> > 
> > For such TSC protected guests, e.g. TDX guests, typically the TSC is
> > configured once at VM level before any vCPU are created and remains
> > unchanged during VM's lifetime.  KVM provides the KVM_SET_TSC_KHZ VM
> > scope ioctl to allow the userspace VMM to configure the TSC of such VM.
> > After that the userspace VMM is not supposed to call the KVM_SET_TSC_KHZ
> > vCPU scope ioctl anymore when creating the vCPU.
> > 
> > The de facto userspace VMM Qemu does this for TDX guests.  The upcoming
> > SEV-SNP guests with Secure TSC should follow.
> > 
> > Note this could be a break of ABI.  But for now only TDX guests are TSC
> > protected and only Qemu supports TDX, thus in practice this should not
> > break any existing userspace.
> > 
> > Suggested-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Kai Huang <kai.huang@...el.com>
> 
> Need to add this in Documentation/virt/kvm/api.rst as well, saying that
> for TDX and SecureTSC enabled SNP guests, KVM_SET_TSC_KHZ vCPU ioctl is
> not valid.
> 
> 

Good point.  Thanks for bringing it up.

I will add below to the doc unless someone has comments?

I'll probably split the doc diff into two parts and merge each to the
respective code change patch, since the change to the doc contains change
to both vm ioctl and vcpu ioctl.

Btw, I think I'll not mention Secure TSC enabled SEV-SNP guests for now
because it is not in upstream yet.  But I tried to make the text in a way
that could be easily extended to cover Secure TSC guests.

diff --git a/Documentation/virt/kvm/api.rst
b/Documentation/virt/kvm/api.rst
index 43ed57e048a8..ad61bcba3791 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -2006,7 +2006,13 @@ 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.  It must be called before any vCPU is created.
+
+For TSC protected CoCo VMs where TSC is configured once at VM scope and
+remains unchanged during VM's lifetime, the VM ioctl should be used to
+configure the TSC and the vCPU ioctl fails.
+
+Example of such CoCo VMs: TDX guests.