Re: [PATCH v8 5/7] kvm: x86: protect masterclock with a seqcount

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Sep 16, 2021 at 06:15:36PM +0000, Oliver Upton wrote:
> From: Paolo Bonzini <pbonzini@redhat.com>
> 
> Protect the reference point for kvmclock with a seqcount, so that
> kvmclock updates for all vCPUs can proceed in parallel.  Xen runstate
> updates will also run in parallel and not bounce the kvmclock cacheline.
> 
> nr_vcpus_matched_tsc is updated outside pvclock_update_vm_gtod_copy
> though, so a spinlock must be kept for that one.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> [Oliver - drop unused locals, don't double acquire tsc_write_lock]
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  7 ++-
>  arch/x86/kvm/x86.c              | 83 +++++++++++++++++----------------
>  2 files changed, 49 insertions(+), 41 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9c34b5b63e39..5accfe7246ce 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1087,6 +1087,11 @@ struct kvm_arch {
>  
>  	unsigned long irq_sources_bitmap;
>  	s64 kvmclock_offset;
> +
> +	/*
> +	 * This also protects nr_vcpus_matched_tsc which is read from a
> +	 * preemption-disabled region, so it must be a raw spinlock.
> +	 */
>  	raw_spinlock_t tsc_write_lock;
>  	u64 last_tsc_nsec;
>  	u64 last_tsc_write;
> @@ -1097,7 +1102,7 @@ struct kvm_arch {
>  	u64 cur_tsc_generation;
>  	int nr_vcpus_matched_tsc;
>  
> -	spinlock_t pvclock_gtod_sync_lock;
> +	seqcount_raw_spinlock_t pvclock_sc;
>  	bool use_master_clock;
>  	u64 master_kernel_ns;
>  	u64 master_cycle_now;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index cb5d5cad5124..29156c49cd11 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2533,9 +2533,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
>  	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
>  
>  	kvm_vcpu_write_tsc_offset(vcpu, offset);
> -	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
>  
> -	spin_lock_irqsave(&kvm->arch.pvclock_gtod_sync_lock, flags);
>  	if (!matched) {
>  		kvm->arch.nr_vcpus_matched_tsc = 0;
>  	} else if (!already_matched) {
> @@ -2543,7 +2541,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
>  	}
>  
>  	kvm_track_tsc_matching(vcpu);
> -	spin_unlock_irqrestore(&kvm->arch.pvclock_gtod_sync_lock, flags);
> +	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
>  }
>  
>  static inline void adjust_tsc_offset_guest(struct kvm_vcpu *vcpu,
> @@ -2731,9 +2729,6 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
>  	int vclock_mode;
>  	bool host_tsc_clocksource, vcpus_matched;
>  
> -	vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
> -			atomic_read(&kvm->online_vcpus));
> -
>  	/*
>  	 * If the host uses TSC clock, then passthrough TSC as stable
>  	 * to the guest.
> @@ -2742,6 +2737,10 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
>  					&ka->master_kernel_ns,
>  					&ka->master_cycle_now);
>  
> +	lockdep_assert_held(&kvm->arch.tsc_write_lock);
> +	vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
> +			atomic_read(&kvm->online_vcpus));
> +
>  	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
>  				&& !ka->backwards_tsc_observed
>  				&& !ka->boot_vcpu_runs_old_kvmclock;
> @@ -2760,14 +2759,18 @@ static void kvm_make_mclock_inprogress_request(struct kvm *kvm)
>  	kvm_make_all_cpus_request(kvm, KVM_REQ_MCLOCK_INPROGRESS);
>  }
>  
> -static void kvm_start_pvclock_update(struct kvm *kvm)
> +static void __kvm_start_pvclock_update(struct kvm *kvm)
>  {
> -	struct kvm_arch *ka = &kvm->arch;
> +	raw_spin_lock_irq(&kvm->arch.tsc_write_lock);
> +	write_seqcount_begin(&kvm->arch.pvclock_sc);
> +}
>  
> +static void kvm_start_pvclock_update(struct kvm *kvm)
> +{
>  	kvm_make_mclock_inprogress_request(kvm);
>  
>  	/* no guest entries from this point */
> -	spin_lock_irq(&ka->pvclock_gtod_sync_lock);
> +	__kvm_start_pvclock_update(kvm);
>  }
>  
>  static void kvm_end_pvclock_update(struct kvm *kvm)
> @@ -2776,7 +2779,8 @@ static void kvm_end_pvclock_update(struct kvm *kvm)
>  	struct kvm_vcpu *vcpu;
>  	int i;
>  
> -	spin_unlock_irq(&ka->pvclock_gtod_sync_lock);
> +	write_seqcount_end(&ka->pvclock_sc);
> +	raw_spin_unlock_irq(&ka->tsc_write_lock);
>  	kvm_for_each_vcpu(i, vcpu, kvm)
>  		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>  
> @@ -2797,20 +2801,12 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
>  {
>  	struct kvm_arch *ka = &kvm->arch;
>  	struct pvclock_vcpu_time_info hv_clock;
> -	unsigned long flags;
>  
> -	spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
>  	if (!ka->use_master_clock) {
> -		spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
>  		data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
>  		return;
>  	}
>  
> -	data->flags |= KVM_CLOCK_TSC_STABLE;
> -	hv_clock.tsc_timestamp = ka->master_cycle_now;
> -	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
> -	spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
> -
>  	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
>  	get_cpu();
>  
> @@ -2825,6 +2821,9 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
>  #endif
>  		data->host_tsc = rdtsc();
>  
> +		data->flags |= KVM_CLOCK_TSC_STABLE;
> +		hv_clock.tsc_timestamp = ka->master_cycle_now;
> +		hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
>  		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>  				   &hv_clock.tsc_shift,
>  				   &hv_clock.tsc_to_system_mul);
> @@ -2839,14 +2838,14 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
>  u64 get_kvmclock_ns(struct kvm *kvm)
>  {
>  	struct kvm_clock_data data;
> +	struct kvm_arch *ka = &kvm->arch;
> +	unsigned seq;
>  
> -	/*
> -	 * Zero flags as it's accessed RMW, leave everything else uninitialized
> -	 * as clock is always written and no other fields are consumed.
> -	 */
> -	data.flags = 0;
> -
> -	get_kvmclock(kvm, &data);
> +	do {
> +		seq = read_seqcount_begin(&ka->pvclock_sc);
> +		data.flags = 0;
> +		get_kvmclock(kvm, &data);
> +	} while (read_seqcount_retry(&ka->pvclock_sc, seq));
>  	return data.clock;
>  }
>  
> @@ -2912,6 +2911,7 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v,
>  static int kvm_guest_time_update(struct kvm_vcpu *v)
>  {
>  	unsigned long flags, tgt_tsc_khz;
> +	unsigned seq;
>  	struct kvm_vcpu_arch *vcpu = &v->arch;
>  	struct kvm_arch *ka = &v->kvm->arch;
>  	s64 kernel_ns;
> @@ -2926,13 +2926,14 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  	 * If the host uses TSC clock, then passthrough TSC as stable
>  	 * to the guest.
>  	 */
> -	spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
> -	use_master_clock = ka->use_master_clock;
> -	if (use_master_clock) {
> -		host_tsc = ka->master_cycle_now;
> -		kernel_ns = ka->master_kernel_ns;
> -	}
> -	spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
> +	seq = read_seqcount_begin(&ka->pvclock_sc);
> +	do {
> +		use_master_clock = ka->use_master_clock;
> +		if (use_master_clock) {
> +			host_tsc = ka->master_cycle_now;
> +			kernel_ns = ka->master_kernel_ns;
> +		}
> +	} while (read_seqcount_retry(&ka->pvclock_sc, seq));
>  
>  	/* Keep irq disabled to prevent changes to the clock */
>  	local_irq_save(flags);
> @@ -5855,10 +5856,15 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
>  
>  static int kvm_vm_ioctl_get_clock(struct kvm *kvm, void __user *argp)
>  {
> -	struct kvm_clock_data data;
> +	struct kvm_clock_data data = { 0 };
> +	unsigned seq;
> +
> +	do {
> +		seq = read_seqcount_begin(&kvm->arch.pvclock_sc);
> +		data.flags = 0;
> +		get_kvmclock(kvm, &data);
> +	} while (read_seqcount_retry(&kvm->arch.pvclock_sc, seq));
>  
> -	memset(&data, 0, sizeof(data));
> -	get_kvmclock(kvm, &data);
>  	if (copy_to_user(argp, &data, sizeof(data)))
>  		return -EFAULT;
>  
> @@ -8159,9 +8165,7 @@ static void kvm_hyperv_tsc_notifier(void)
>  	kvm_max_guest_tsc_khz = tsc_khz;
>  
>  	list_for_each_entry(kvm, &vm_list, vm_list) {
> -		struct kvm_arch *ka = &kvm->arch;
> -
> -		spin_lock_irq(&ka->pvclock_gtod_sync_lock);
> +		__kvm_start_pvclock_update(kvm);
>  		pvclock_update_vm_gtod_copy(kvm);
>  		kvm_end_pvclock_update(kvm);
>  	}
> @@ -11188,8 +11192,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  
>  	raw_spin_lock_init(&kvm->arch.tsc_write_lock);
>  	mutex_init(&kvm->arch.apic_map_lock);
> -	spin_lock_init(&kvm->arch.pvclock_gtod_sync_lock);
> -
> +	seqcount_raw_spinlock_init(&kvm->arch.pvclock_sc, &kvm->arch.tsc_write_lock);
>  	kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
>  	pvclock_update_vm_gtod_copy(kvm);
>  
> -- 
> 2.33.0.309.g3052b89438-goog
> 
> 

ACK


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