linux-kernel - RE: [kvm-devel] [PATCH 2/3] kvmclock

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Message-ID: <10EA09EFD8728347A513008B6B0DA77A025DFCCF@pdsmsx411.ccr.corp.intel.com>
Date:	Wed, 14 Nov 2007 08:41:16 +0800
From:	"Dong, Eddie" <eddie.dong@...el.com>
To:	"Avi Kivity" <avi@...ranet.com>
Cc:	"Glauber de Oliveira Costa" <gcosta@...hat.com>,
	<linux-kernel@...r.kernel.org>, <jeremy@...p.org>,
	<hollisb@...ibm.com>, <kvm-devel@...ts.sourceforge.net>
Subject: RE: [kvm-devel] [PATCH 2/3] kvmclock - the host part.

>> 
>> +	kvm_get_msr(vcpu, MSR_IA32_TIME_STAMP_COUNTER,
>> +			  &vcpu->hv_clock.last_tsc);
>> +
>> +	ktime_get_ts(&ts);
>> +	vcpu->hv_clock.now_ns = ts.tv_nsec + (NSEC_PER_SEC *
>> (u64)ts.tv_sec); +	vcpu->hv_clock.wc_sec = get_seconds();
>> 
>> I am even thinking we have to disable interrupt between these lines,
>> otherwise guest wall clock may see backward time source when
>> calculating the delta TSC since last vcpu->hv_clock.now_ns update.
>> 
> 
> That's true.  While we do need to handle vcpu migration and
> descheduling, the code sequence you note needs to be as atomic as
> possible. 
> 

Yes VCPU migration is another issue. Since the physical platform TSC is 
totally different across migration, so we can't expose host TSC to
guest.
For KVM, it should be simple to just use guest_TSC (=HOST_TSC + offset).

thanks, eddie
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