linux-kernel - Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen

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Message-ID: <20130506143533.GF15278@phenom.dumpdata.com>
Date:	Mon, 6 May 2013 10:35:33 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Christopher Covington <cov@...eaurora.org>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"will.deacon@....com" <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce
 arch_timer_stolen_ticks

> > e.g. if a VCPU sets a timer for NOW+5, but 3 are stolen in the middle it
> > would not make sense (from the guests PoV) for NOW'==NOW+2 at the point
> > where the timer goes off. Nor does it make sense to require that the
> > guest actually be running for 5 before injecting the timer because that
> > would mean real time elapsed time for the timer would be 5+3 in the case
> > where 3 are stolen.
> 
> This is a bit of an aside, but I think that hiding time spent at higher
> privilege levels can be a quite sensible approach to timekeeping in a
> virtualized environment, but I understand that it's not the approach taken
> with Xen, and as you pointed out above, adjusting the Virtual Offset Register
> by itself isn't enough to implement that approach.

This is the approach taken by Xen and KVM. Look in CONFIG_PARAVIRT_CLOCK for
implementation. In the user-space, the entry in 'top' of "stolen" (%st)
is for this exact value.
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