Re: [PATCH 1/2] KVM: x86: reduce pvclock_gtod_sync_lock critical sections

[Paolo Bonzini <pbonzini@redhat.com>]

On 07/04/21 19:40, Marcelo Tosatti wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index fe806e894212..0a83eff40b43 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -2562,10 +2562,12 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
>>   
>>   	kvm_hv_invalidate_tsc_page(kvm);
>>   
>> -	spin_lock(&ka->pvclock_gtod_sync_lock);
>>   	kvm_make_mclock_inprogress_request(kvm);
>> +
> Might be good to serialize against two kvm_gen_update_masterclock
> callers? Otherwise one caller could clear KVM_REQ_MCLOCK_INPROGRESS,
> while the other is still at pvclock_update_vm_gtod_copy().

Makes sense, but this stuff has always seemed unnecessarily complicated 
to me.

KVM_REQ_MCLOCK_INPROGRESS is only needed to kick running vCPUs out of 
the execution loop; clearing it in kvm_gen_update_masterclock is 
unnecessary, because KVM_REQ_CLOCK_UPDATE takes pvclock_gtod_sync_lock 
too and thus will already wait for pvclock_update_vm_gtod_copy to end.

I think it's possible to use a seqcount in KVM_REQ_CLOCK_UPDATE instead 
of KVM_REQ_MCLOCK_INPROGRESS.  Both cause the vCPUs to spin. I'll take a 
look.

Paolo