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

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Message-ID: <2abe4b19-e41e-34f9-0a3c-30812c7b719e@redhat.com>
Date:   Thu, 8 Apr 2021 14:25:41 +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 08/04/21 14:00, Marcelo Tosatti wrote:
>>
>> 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.

Yes this is what KVM_REQ_MCLOCK_INPROGRESS does, but it does not have to 
be done that way.  All you really need is the IPI with KVM_REQUEST_WAIT, 
which ensures that updates happen after the vCPUs have exited guest 
mode.  You don't need to loop on vcpu->requests for example, because 
kvm_guest_time_update could just spin on pvclock_gtod_sync_lock until 
pvclock_update_vm_gtod_copy is done.

So this morning I tried protecting the kvm->arch fields for kvmclock 
using a seqcount, which is nice also because get_kvmclock_ns() does not 
have to bounce the cacheline of pvclock_gtod_sync_lock anymore.  I'll 
post it tomorrow or next week.

Paolo