linux-kernel - Re: [PATCH 1/2] KVM: x86: reduce pvclock_gtod_sync

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Message-ID: <51cae826-8973-5113-7e12-8163eab36cb7@redhat.com>
Date:   Thu, 8 Apr 2021 10:15:16 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, dwmw@...zon.co.uk
Subject: Re: [PATCH 1/2] KVM: x86: reduce pvclock_gtod_sync_lock critical
 sections

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