Re: [PATCH v4] KVM: x86/tsc: Don't sync user changes to TSC with KVM-initiated change

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 13, 2023, David Woodhouse wrote:
> On Fri, 2023-08-11 at 15:59 -0700, Sean Christopherson wrote:
> > On Tue, Aug 01, 2023, Like Xu wrote:
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 278dbd37dab2..eeaf4ad9174d 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -2713,7 +2713,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
> > >         kvm_track_tsc_matching(vcpu);
> > >   }
> > >   
> > > -static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
> > > +static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data, bool user_initiated)
> > 
> > Rather than pass two somewhat magic values for the KVM-internal call, what about
> > making @data a pointer and passing NULL?
> 
> Why change that at all?
> 
> Userspace used to be able to force a sync by writing zero. You are
> removing that from the ABI without any explanation about why;

No, my suggestion did not remove that from the ABI.  A @user_value of '0' would
still force synchronization.

-static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
+static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
 {
+       u64 data = user_value ? *user_value : 0;  <=== "*user_value" is '0'
        struct kvm *kvm = vcpu->kvm;
        u64 offset, ns, elapsed;
        unsigned long flags;
@@ -2712,14 +2713,17 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
        elapsed = ns - kvm->arch.last_tsc_nsec;

        if (vcpu->arch.virtual_tsc_khz) {
+               /*
+                * Force synchronization when creating or hotplugging a vCPU,
+                * i.e. when the TSC value is '0', to help keep clocks stable.
+                * If this is NOT a hotplug/creation case, skip synchronization
+                * on the first write from userspace so as not to misconstrue
+                * state restoration after live migration as an attempt from
+                * userspace to synchronize.
+                */
                if (data == 0) { <== "data" still '0', still forces synchronization
-                       /*
-                        * detection of vcpu initialization -- need to sync
-                        * with other vCPUs. This particularly helps to keep
-                        * kvm_clock stable after CPU hotplug
-                        */
                        synchronizing = true;

> it doesn't seem necessary for fixing the original issue.

It's necessary for "user_set_tsc" to be an accurate name.  The code in v6 yields
"user_set_tsc_to_non_zero_value".  And I don't think it's just a naming issue,
e.g. if userspace writes '0' immediately after creating, and then later writes a
small delta, the v6 code wouldn't trigger synchronization because "user_set_tsc"
would be left unseft by the write of '0'.