linux-kernel - Re: [GIT PULL] First batch of KVM changes for 4.1

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Message-ID: <20150417134335.GI23123@twins.programming.kicks-ass.net>
Date:	Fri, 17 Apr 2015 15:43:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	gleb@...nel.org, kvm@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>, mtosatti@...hat.com,
	luto@...nel.org
Subject: Re: [GIT PULL] First batch of KVM changes for 4.1

On Fri, Apr 17, 2015 at 03:38:58PM +0200, Paolo Bonzini wrote:
> > The path this notifier is called from has nothing to do with those
> > costs.
> 
> How not?  The task is going to incur those costs, it's not like half
> a dozen extra instruction make any difference.  But anyway...

Its attributed to the entity doing the migration, which can be the
wakeup path or a softirq. And we very much do care about the wakeup
path.

> ... that's a valid objection.  Please look at the patch below.

Still a NAK on that, distros have no choice but to enable that CONFIG
option because people might want to run KVM.

CONFIG options are pointless if they end up being mandatory.
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