linux-kernel - Re: [PATCH v2 0/4] implement vcpu preempted check

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Message-ID: <20160706115954.GK30921@twins.programming.kicks-ass.net>
Date:	Wed, 6 Jul 2016 13:59:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-s390@...r.kernel.org, dave@...olabs.net, mpe@...erman.id.au,
	boqun.feng@...il.com, will.deacon@....com,
	linux-kernel@...r.kernel.org, waiman.long@....com,
	virtualization@...ts.linux-foundation.org, mingo@...hat.com,
	paulus@...ba.org, benh@...nel.crashing.org, schwidefsky@...ibm.com,
	paulmck@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 0/4] implement vcpu preempted check

On Wed, Jul 06, 2016 at 12:44:58PM +0200, Paolo Bonzini wrote:
> > Paolo, could you help out with an (x86) KVM interface for this?
> 
> If it's just for spin loops, you can check if the version field in the
> steal time structure has changed.

That would require remembering the old value, no?

That would work with a previous interface proposal, see:

  http://lkml.kernel.org/r/1466937715-6683-2-git-send-email-xinhui.pan@linux.vnet.ibm.com

the vcpu_get_yield_count() thing would match that I think.

However the current proposal:

  http://lkml.kernel.org/r/1467124991-13164-2-git-send-email-xinhui.pan@linux.vnet.ibm.com

dropped that in favour of only vcpu_is_preempted(), which requires being
able to tell if a (remote) vcpu is currently running or not, which iirc,
isn't possible with the steal time sequence count.