linux-kernel - Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

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Date:	Mon, 05 Jan 2015 23:23:41 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso
 pvclock reader



On 05/01/2015 19:56, Andy Lutomirski wrote:
>> > 1) State: all pvtis marked as PVCLOCK_TSC_STABLE_BIT.
>> > 1) Update request for all vcpus, for a TSC_STABLE_BIT -> ~TSC_STABLE_BIT
>> > transition.
>> > 2) vCPU-1 updates its pvti with new values.
>> > 3) vCPU-0 still has not updated its pvti with new values.
>> > 4) vCPU-1 VM-enters, uses vCPU-0 values, even though it has been
>> > notified of a TSC_STABLE_BIT -> ~TSC_STABLE_BIT transition.
>> >
>> > The update is not actually atomic across all vCPUs, its atomic in
>> > the sense of not allowing visibility of distinct
>> > system_timestamp/tsc_timestamp values.
>> >
> Hmm.  In step 4, is there a guarantee that vCPU-0 won't VM-enter until
> it gets marked unstable?  Otherwise the vdso could could just as
> easily be called from vCPU-1, migrated to vCPU-0, read the data
> complete with stale stable bit, and get migrated back to vCPU-1.
> 
> But I thought that KVM currently froze all vCPUs when updating pvti
> for any of them.  How can this happen?  I admit I don't really
> understand the update request code.

That was also my understanding.  I thought this was the point of
kvm_make_mclock_inprogress_request/KVM_REQ_MCLOCK_INPROGRESS.

Disabling TSC_STABLE_BIT is triggered by pvclock_gtod_update_fn but it
happens in kvm_gen_update_masterclock, and no guest entries will happen
in the meanwhile.

Paolo
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