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

[Paolo Bonzini]

On 16/09/21 20:15, Oliver Upton wrote:
> From: Paolo Bonzini<pbonzini at 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 at redhat.com>
> [Oliver - drop unused locals, don't double acquire tsc_write_lock]
> Signed-off-by: Oliver Upton<oupton at google.com>
> ---

This needs a small adjustment:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 07d00e711043..b0c21d42f453 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11289,6 +11289,7 @@ void kvm_arch_free_vm(struct kvm *kvm)
  int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
  {
  	int ret;
+	unsigned long flags;
  
  	if (type)
  		return -EINVAL;
@@ -11314,7 +11315,10 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
  	mutex_init(&kvm->arch.apic_map_lock);
  	seqcount_raw_spinlock_init(&kvm->arch.pvclock_sc, &kvm->arch.tsc_write_lock);
  	kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
+
+	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
  	pvclock_update_vm_gtod_copy(kvm);
+	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
  
  	kvm->arch.guest_can_read_msr_platform_info = true;