Date:   Mon, 7 Jun 2021 10:25:45 +0000
From:   "Stamatis, Ilias" <ilstam@...zon.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>
CC:     "jmattson@...gle.com" <jmattson@...gle.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "zamsden@...il.com" <zamsden@...il.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [PATCH v5 09/11] KVM: X86: Add vendor callbacks for writing the TSC
 multiplier

On Sun, 2021-05-30 at 12:51 +0300, Maxim Levitsky wrote:
> On Fri, 2021-05-28 at 11:57 +0100, Ilias Stamatis wrote:
> > Currently vmx_vcpu_load_vmcs() writes the TSC_MULTIPLIER field of the
> > VMCS every time the VMCS is loaded. Instead of doing this, set this
> > field from common code on initialization and whenever the scaling ratio
> > changes.
> > 
> > Additionally remove vmx->current_tsc_ratio. This field is redundant as
> > vcpu->arch.tsc_scaling_ratio already tracks the current TSC scaling
> > ratio. The vmx->current_tsc_ratio field is only used for avoiding
> > unnecessary writes but it is no longer needed after removing the code
> > from the VMCS load path.
> > 
> > Suggested-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
> > ---
> >  arch/x86/include/asm/kvm-x86-ops.h |  1 +
> >  arch/x86/include/asm/kvm_host.h    |  1 +
> >  arch/x86/kvm/svm/svm.c             |  6 ++++++
> >  arch/x86/kvm/vmx/nested.c          |  9 ++++-----
> >  arch/x86/kvm/vmx/vmx.c             | 11 ++++++-----
> >  arch/x86/kvm/vmx/vmx.h             |  8 --------
> >  arch/x86/kvm/x86.c                 | 30 +++++++++++++++++++++++-------
> >  7 files changed, 41 insertions(+), 25 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index 029c9615378f..34ad7a17458a 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -90,6 +90,7 @@ KVM_X86_OP_NULL(has_wbinvd_exit)
> >  KVM_X86_OP(get_l2_tsc_offset)
> >  KVM_X86_OP(get_l2_tsc_multiplier)
> >  KVM_X86_OP(write_tsc_offset)
> > +KVM_X86_OP(write_tsc_multiplier)
> >  KVM_X86_OP(get_exit_info)
> >  KVM_X86_OP(check_intercept)
> >  KVM_X86_OP(handle_exit_irqoff)
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index f099277b993d..a334ce7741ab 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1308,6 +1308,7 @@ struct kvm_x86_ops {
> >       u64 (*get_l2_tsc_offset)(struct kvm_vcpu *vcpu);
> >       u64 (*get_l2_tsc_multiplier)(struct kvm_vcpu *vcpu);
> >       void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
> > +     void (*write_tsc_multiplier)(struct kvm_vcpu *vcpu, u64 multiplier);
> > 
> >       /*
> >        * Retrieve somewhat arbitrary exit information.  Intended to be used
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 8dfb2513b72a..cb701b42b08b 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -1103,6 +1103,11 @@ static void svm_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> >       vmcb_mark_dirty(svm->vmcb, VMCB_INTERCEPTS);
> >  }
> > 
> > +static void svm_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 multiplier)
> > +{
> > +     wrmsrl(MSR_AMD64_TSC_RATIO, multiplier);
> > +}
> > +
> >  /* Evaluate instruction intercepts that depend on guest CPUID features. */
> >  static void svm_recalc_instruction_intercepts(struct kvm_vcpu *vcpu,
> >                                             struct vcpu_svm *svm)
> > @@ -4528,6 +4533,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
> >       .get_l2_tsc_offset = svm_get_l2_tsc_offset,
> >       .get_l2_tsc_multiplier = svm_get_l2_tsc_multiplier,
> >       .write_tsc_offset = svm_write_tsc_offset,
> > +     .write_tsc_multiplier = svm_write_tsc_multiplier,
> > 
> >       .load_mmu_pgd = svm_load_mmu_pgd,
> > 
> > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > index 6058a65a6ede..239154d3e4e7 100644
> > --- a/arch/x86/kvm/vmx/nested.c
> > +++ b/arch/x86/kvm/vmx/nested.c
> > @@ -2533,9 +2533,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> >       }
> > 
> >       vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> > -
> >       if (kvm_has_tsc_control)
> > -             decache_tsc_multiplier(vmx);
> > +             vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
> 
> We still end up writing the TSC_MULTIPLIER in the vmcs02 on each nested VM entry
> almost always for nothing since for the vast majority of the entries we will
> write the same value.
> 

Yes, I already addressed this but as per Sean's response this should be alright
for now and it can be "fixed" in a separate series. 

This patch completely removes the vmwrite from the VMCS01 load path which is
called more frequently anyway.

> It is probably OK for now to leave it like that, and then add some sort of 'dirty' tracking
> to track if the userspace or L1 changed the TSC multiplier for L2 (L1 writes to vmcb12
> are tracked by using the 'dirty_vmcs' flag, assuming that we don't shadow TSC_MULTIPLIER field)
> 
> So the above should later go to prepare_vmcs02_rare, and it should also be done if
> host TSC multiplier changed (not a problem IMHO to have another piece of code doing that).
> 
> 
> > 
> >       nested_vmx_transition_tlb_flush(vcpu, vmcs12, true);
> > 
> > @@ -4501,12 +4500,12 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
> >       vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
> >       vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
> >       vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> > +     if (kvm_has_tsc_control)
> > +             vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
> 
> This I think isn't needed, since this write is done after we have already
> switched to vmcs01, and it should have the L1 value it had prior
> to the nested entry.
> 

What if userspace changed L1's multiplier while L2 was active?

> 
> > +
> >       if (vmx->nested.l1_tpr_threshold != -1)
> >               vmcs_write32(TPR_THRESHOLD, vmx->nested.l1_tpr_threshold);
> > 
> > -     if (kvm_has_tsc_control)
> > -             decache_tsc_multiplier(vmx);
> > -
> >       if (vmx->nested.change_vmcs01_virtual_apic_mode) {
> >               vmx->nested.change_vmcs01_virtual_apic_mode = false;
> >               vmx_set_virtual_apic_mode(vcpu);
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 4b70431c2edd..bf845a08995e 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1390,11 +1390,6 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
> > 
> >               vmx->loaded_vmcs->cpu = cpu;
> >       }
> > -
> > -     /* Setup TSC multiplier */
> > -     if (kvm_has_tsc_control &&
> > -         vmx->current_tsc_ratio != vcpu->arch.tsc_scaling_ratio)
> > -             decache_tsc_multiplier(vmx);
> >  }
> > 
> >  /*
> > @@ -1813,6 +1808,11 @@ static void vmx_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> >       vmcs_write64(TSC_OFFSET, offset);
> >  }
> > 
> > +static void vmx_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 multiplier)
> > +{
> > +     vmcs_write64(TSC_MULTIPLIER, multiplier);
> > +}
> > +
> >  /*
> >   * nested_vmx_allowed() checks whether a guest should be allowed to use VMX
> >   * instructions and MSRs (i.e., nested VMX). Nested VMX is disabled for
> > @@ -7707,6 +7707,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
> >       .get_l2_tsc_offset = vmx_get_l2_tsc_offset,
> >       .get_l2_tsc_multiplier = vmx_get_l2_tsc_multiplier,
> >       .write_tsc_offset = vmx_write_tsc_offset,
> > +     .write_tsc_multiplier = vmx_write_tsc_multiplier,
> > 
> >       .load_mmu_pgd = vmx_load_mmu_pgd,
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index aa97c82e3451..3eaa86a0ba3e 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -322,8 +322,6 @@ struct vcpu_vmx {
> >       /* apic deadline value in host tsc */
> >       u64 hv_deadline_tsc;
> > 
> > -     u64 current_tsc_ratio;
> > -
> >       unsigned long host_debugctlmsr;
> > 
> >       /*
> > @@ -532,12 +530,6 @@ static inline struct vmcs *alloc_vmcs(bool shadow)
> >                             GFP_KERNEL_ACCOUNT);
> >  }
> > 
> > -static inline void decache_tsc_multiplier(struct vcpu_vmx *vmx)
> > -{
> > -     vmx->current_tsc_ratio = vmx->vcpu.arch.tsc_scaling_ratio;
> > -     vmcs_write64(TSC_MULTIPLIER, vmx->current_tsc_ratio);
> > -}
> > -
> >  static inline bool vmx_has_waitpkg(struct vcpu_vmx *vmx)
> >  {
> >       return vmx->secondary_exec_control &
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 801fa1e8e915..c1e14dadad2d 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -2179,14 +2179,15 @@ static u32 adjust_tsc_khz(u32 khz, s32 ppm)
> >       return v;
> >  }
> > 
> > +static void kvm_vcpu_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 l1_multiplier);
> > +
> >  static int set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale)
> >  {
> >       u64 ratio;
> > 
> >       /* Guest TSC same frequency as host TSC? */
> >       if (!scale) {
> > -             vcpu->arch.l1_tsc_scaling_ratio = kvm_default_tsc_scaling_ratio;
> > -             vcpu->arch.tsc_scaling_ratio = kvm_default_tsc_scaling_ratio;
> > +             kvm_vcpu_write_tsc_multiplier(vcpu, kvm_default_tsc_scaling_ratio);
> >               return 0;
> >       }
> > 
> > @@ -2212,7 +2213,7 @@ static int set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale)
> >               return -1;
> >       }
> > 
> > -     vcpu->arch.l1_tsc_scaling_ratio = vcpu->arch.tsc_scaling_ratio = ratio;
> > +     kvm_vcpu_write_tsc_multiplier(vcpu, ratio);
> >       return 0;
> >  }
> > 
> > @@ -2224,8 +2225,7 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz)
> >       /* tsc_khz can be zero if TSC calibration fails */
> >       if (user_tsc_khz == 0) {
> >               /* set tsc_scaling_ratio to a safe value */
> > -             vcpu->arch.l1_tsc_scaling_ratio = kvm_default_tsc_scaling_ratio;
> > -             vcpu->arch.tsc_scaling_ratio = kvm_default_tsc_scaling_ratio;
> > +             kvm_vcpu_write_tsc_multiplier(vcpu, kvm_default_tsc_scaling_ratio);
> >               return -1;
> >       }
> > 
> > @@ -2383,6 +2383,23 @@ static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 l1_offset)
> >       static_call(kvm_x86_write_tsc_offset)(vcpu, vcpu->arch.tsc_offset);
> >  }
> > 
> > +static void kvm_vcpu_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 l1_multiplier)
> > +{
> > +     vcpu->arch.l1_tsc_scaling_ratio = l1_multiplier;
> > +
> > +     /* Userspace is changing the multiplier while L2 is active */
> 
> Nitpick about the comment:
> On SVM, the TSC multiplier is a MSR, so a crazy L1 can give L2 access to it,
> so L2 can in theory change its TSC multiplier as well
> (I am not sure if this is even allowed by the SVM spec)
> 

If L1 chooses not to trap a WRMSR to the multiplier this should still change
L1's multiplier, not L2's, right? 

It's the exact same case we have in kvm_vcpu_write_tsc_offset() too and the 
comment there addresses exactly this.

> > +     if (is_guest_mode(vcpu))
> > +             vcpu->arch.tsc_scaling_ratio = kvm_calc_nested_tsc_multiplier(
> > +                     l1_multiplier,
> > +                     static_call(kvm_x86_get_l2_tsc_multiplier)(vcpu));
> > +     else
> > +             vcpu->arch.tsc_scaling_ratio = l1_multiplier;
> > +
> > +     if (kvm_has_tsc_control)
> > +             static_call(kvm_x86_write_tsc_multiplier)(
> > +                     vcpu, vcpu->arch.tsc_scaling_ratio);
> > +}
> > +
> >  static inline bool kvm_check_tsc_unstable(void)
> >  {
> >  #ifdef CONFIG_X86_64
> > @@ -10343,8 +10360,6 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >       else
> >               vcpu->arch.mp_state = KVM_MP_STATE_UNINITIALIZED;
> > 
> > -     kvm_set_tsc_khz(vcpu, max_tsc_khz);
> > -
> >       r = kvm_mmu_create(vcpu);
> >       if (r < 0)
> >               return r;
> > @@ -10443,6 +10458,7 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
> >       if (mutex_lock_killable(&vcpu->mutex))
> >               return;
> >       vcpu_load(vcpu);
> > +     kvm_set_tsc_khz(vcpu, max_tsc_khz);
> >       kvm_synchronize_tsc(vcpu, 0);
> >       vcpu_put(vcpu);
> > 
> 
> 
> Best regards,
>         Maxim Levitsky
> 
> 

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