linux-kernel - Re: Cleaning up the KVM clock

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Date:	Tue, 23 Dec 2014 11:25:08 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Santosh Shukla <sshukla@...sta.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Cleaning up the KVM clock



On 23/12/2014 11:23, Santosh Shukla wrote:
> 
> 
>     No.  kvm_guest_time_update is called by vcpu_enter_guest, while the vCPU
>     is not running, so it's entirely atomic from the point of view of
>     the guest.
> 
>  
> Then checking odd value for version field (at guest side: function
> pvclock_clocksource_read / pvclock_read_flag) is redundant considering
> that  kvm_guest_time_update incremented by 2. 

The code is common to Xen and KVM.  Xen uses seqlock semantics.  The
cost of one AND is not detectable.

Paolo
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