linux-kernel - Re: [PATCH v1 0/5] KVM in-guest performance monitoring

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Date:	Thu, 12 May 2011 15:43:40 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Avi Kivity <avi@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v1 0/5] KVM in-guest performance monitoring

On Thu, May 12, 2011 at 03:23:39PM +0200, Jan Kiszka wrote:
> On 2011-05-12 15:11, Joerg Roedel wrote:

> > Seriously, I think such decisions should be technical only and not
> > political like that. The losers of such political decisions are always
> > the users because they don't get useful features that are technical
> > possible.
> 
> Paravirt remains a workaround, useful until hardware provides a solution
> for all guests, and that often in an even more efficient way (like for
> MMU virtualization).

Fully agreed. And todays x86 CPUs lack proper support for virtualizing
the PMU. That will hopefully change but users want the feature today.

> We do not need to block a PV-PMU for Linux guests (or other OSes that
> want to adopt to it), but that will not be a solution for the problem,
> that's my point. A PV-PMU may even be useful to demonstrate usefulness
> of a virtual PMU the CPU vendors (if they aren't aware of this yet).

Right, if users actually use the virtual PMU this probably increases the
priority for proper hardware support.

	Joerg

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