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

On 16/09/21 20:15, 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<redacted>
---

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;
 

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