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

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Date:   Thu, 8 Apr 2021 09:00:21 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...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

Hi Paolo,

On Thu, Apr 08, 2021 at 10:15:16AM +0200, Paolo Bonzini wrote:
> 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; 

We do not want vcpus with different system_timestamp/tsc_timestamp
pair:

 * To avoid that problem, do not allow visibility of distinct
 * system_timestamp/tsc_timestamp values simultaneously: use a master
 * copy of host monotonic time values. Update that master copy
 * in lockstep.

So KVM_REQ_MCLOCK_INPROGRESS also ensures that no vcpu enters 
guest mode (via vcpu->requests check before VM-entry) with a 
different system_timestamp/tsc_timestamp pair.

> 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