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

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Message-ID: <e3da1c7e-fa47-415f-99bf-f372057f0a75@amd.com>
Date: Wed, 9 Jul 2025 14:09:23 +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,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC
 protected guest

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.

> ---
>  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 2806f7104295..699ca5e74bba 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6186,6 +6186,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  		u32 user_tsc_khz;
>  
>  		r = -EINVAL;
> +
> +		if (vcpu->arch.guest_tsc_protected)
> +			goto out;
> +
>  		user_tsc_khz = (u32)arg;
>  
>  		if (kvm_caps.has_tsc_control &&

Regards
Nikunj