Re: [PATCH v2 03/10] KVM: X86: Add kvm_scale_tsc_l1() and kvm_compute_tsc_offset_l1()

["Stamatis, Ilias" <ilstam@amazon.com>]

On Tue, 2021-05-18 at 23:04 +0000, Sean Christopherson wrote:
> On Wed, May 12, 2021, Ilias Stamatis wrote:
> > The existing kvm_scale_tsc() scales the TSC using the current TSC
> > scaling ratio. That used to be the same as L1's scaling ratio but now
> > with nested TSC scaling support it is no longer the case.
> > 
> > This patch adds a new kvm_scale_tsc_l1() function that scales the TSC
> > using L1's scaling ratio. The existing kvm_scale_tsc() can still be used
> > for scaling L2 TSC values.
> > 
> > Additionally, this patch renames the kvm_compute_tsc_offset() function
> > to kvm_compute_tsc_offset_l1() and has the function treat its TSC
> > argument as an L1 TSC value. All existing code uses this function
> > passing L1 values to it.
> > 
> > Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  1 +
> >  arch/x86/kvm/x86.c              | 41 ++++++++++++++++++++++-----------
> >  2 files changed, 29 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 7dfc609eacd6..be59197e5eb7 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1789,6 +1789,7 @@ static inline bool kvm_is_supported_user_return_msr(u32 msr)
> >  }
> > 
> >  u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc);
> > +u64 kvm_scale_tsc_l1(struct kvm_vcpu *vcpu, u64 tsc);
> >  u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc);
> 
> I don't really care which version is used, but we should be consistent, i.e. choose
> kvm_<action>_tsc_l1 or kvm_<action>_tsc_l1, not both.  The easy choice is the
> former since it's already there.

OK

> 
> >  unsigned long kvm_get_linear_rip(struct kvm_vcpu *vcpu);
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 07cf5d7ece38..84af1af7a2cc 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -2319,18 +2319,30 @@ u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc)
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_scale_tsc);
> > 
> > -static u64 kvm_compute_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
> > +u64 kvm_scale_tsc_l1(struct kvm_vcpu *vcpu, u64 tsc)
> > +{
> > +     u64 _tsc = tsc;
> > +     u64 ratio = vcpu->arch.l1_tsc_scaling_ratio;
> > +
> > +     if (ratio != kvm_default_tsc_scaling_ratio)
> > +             _tsc = __scale_tsc(ratio, tsc);
> > +
> > +     return _tsc;
> > +}
> 
> Just make the ratio a param.  This is complete copy+paste of kvm_scale_tsc(),
> with 3 characters added.  And all of the callers are already in an L1-specific
> function or have L1 vs. L2 awareness.  IMO, that makes the code less magical, too,
> as I don't have to dive into a helper to see that it reads l1_tsc_scaling_ratio
> versus tsc_scaling_ratio.
> 

That's how I did it initially but changed it into a separate function after
receiving feedback on v1. I'm neutral, I don't mind changing it back.

More
opinions?

> > +EXPORT_SYMBOL_GPL(kvm_scale_tsc_l1);
> > +
> > +static u64 kvm_compute_tsc_offset_l1(struct kvm_vcpu *vcpu, u64 target_tsc)
> >  {
> >       u64 tsc;
> > 
> > -     tsc = kvm_scale_tsc(vcpu, rdtsc());
> > +     tsc = kvm_scale_tsc_l1(vcpu, rdtsc());
> > 
> >       return target_tsc - tsc;
> >  }
> > 
> >  u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
> >  {
> > -     return vcpu->arch.l1_tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> > +     return vcpu->arch.l1_tsc_offset + kvm_scale_tsc_l1(vcpu, host_tsc);
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
> > 
> > @@ -2363,7 +2375,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
> >       bool synchronizing = false;
> > 
> >       raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
> > -     offset = kvm_compute_tsc_offset(vcpu, data);
> > +     offset = kvm_compute_tsc_offset_l1(vcpu, data);
> >       ns = get_kvmclock_base_ns();
> >       elapsed = ns - kvm->arch.last_tsc_nsec;
> > 
> > @@ -2402,7 +2414,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
> >               } else {
> >                       u64 delta = nsec_to_cycles(vcpu, elapsed);
> >                       data += delta;
> > -                     offset = kvm_compute_tsc_offset(vcpu, data);
> > +                     offset = kvm_compute_tsc_offset_l1(vcpu, data);
> >               }
> >               matched = true;
> >               already_matched = (vcpu->arch.this_tsc_generation == kvm->arch.cur_tsc_generation);
> > @@ -2463,7 +2475,7 @@ static inline void adjust_tsc_offset_host(struct kvm_vcpu *vcpu, s64 adjustment)
> >  {
> >       if (vcpu->arch.l1_tsc_scaling_ratio != kvm_default_tsc_scaling_ratio)
> >               WARN_ON(adjustment < 0);
> > -     adjustment = kvm_scale_tsc(vcpu, (u64) adjustment);
> > +     adjustment = kvm_scale_tsc_l1(vcpu, (u64) adjustment);
> >       adjust_tsc_offset_guest(vcpu, adjustment);
> >  }
> > 
> > @@ -2846,7 +2858,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> >       /* With all the info we got, fill in the values */
> > 
> >       if (kvm_has_tsc_control)
> > -             tgt_tsc_khz = kvm_scale_tsc(v, tgt_tsc_khz);
> > +             tgt_tsc_khz = kvm_scale_tsc_l1(v, tgt_tsc_khz);
> > 
> >       if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
> >               kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
> > @@ -3235,7 +3247,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >               if (msr_info->host_initiated) {
> >                       kvm_synchronize_tsc(vcpu, data);
> >               } else {
> > -                     u64 adj = kvm_compute_tsc_offset(vcpu, data) - vcpu->arch.l1_tsc_offset;
> > +                     u64 adj = kvm_compute_tsc_offset_l1(vcpu, data) - vcpu->arch.l1_tsc_offset;
> >                       adjust_tsc_offset_guest(vcpu, adj);
> >                       vcpu->arch.ia32_tsc_adjust_msr += adj;
> >               }
> > @@ -3537,10 +3549,13 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >                * return L1's TSC value to ensure backwards-compatible
> >                * behavior for migration.
> >                */
> > -             u64 tsc_offset = msr_info->host_initiated ? vcpu->arch.l1_tsc_offset :
> > -                                                         vcpu->arch.tsc_offset;
> > -
> > -             msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) + tsc_offset;
> > +             if (msr_info->host_initiated) {
> 
> Unnecessary curly braces.
> 
> > +                     msr_info->data = kvm_scale_tsc_l1(vcpu, rdtsc()) +
> > +                                      vcpu->arch.l1_tsc_offset;
> > +             } else {
> > +                     msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) +
> > +                                      vcpu->arch.tsc_offset;
> > +             }
> >               break;
> >       }
> >       case MSR_MTRRcap:
> > @@ -4123,7 +4138,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >                       mark_tsc_unstable("KVM discovered backwards TSC");
> > 
> >               if (kvm_check_tsc_unstable()) {
> > -                     u64 offset = kvm_compute_tsc_offset(vcpu,
> > +                     u64 offset = kvm_compute_tsc_offset_l1(vcpu,
> >                                               vcpu->arch.last_guest_tsc);
> >                       kvm_vcpu_write_tsc_offset(vcpu, offset);
> >                       vcpu->arch.tsc_catchup = 1;
> > --
> > 2.17.1
> >