Re: [PATCH 2/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPU has been created

["Huang, Kai" <kai.huang@intel.com>]

On Thu, 2025-07-10 at 22:53 +0000, Huang, Kai wrote:
> 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@google.com>
> > > > Signed-off-by: Kai Huang <kai.huang@intel.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.

Sorry for multiple emails, but I ended up with below.

AFAICT the actual updating of kvm->arch.default_tsc_khz needs to be in the
kvm->lock mutex too.  Please let me know if you found any issue?

diff --git a/Documentation/virt/kvm/api.rst
b/Documentation/virt/kvm/api.rst
index 43ed57e048a8..86ea1e2b2737 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.  It 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: {