linux-kernel - Re: [PATCH 3/7] KVM-HV: KVM Steal time implementation

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Message-ID: <20110619125956.GV491@redhat.com>
Date:	Sun, 19 Jun 2011 15:59:56 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH 3/7] KVM-HV: KVM Steal time implementation

On Sun, Jun 19, 2011 at 03:35:58PM +0300, Avi Kivity wrote:
> On 06/15/2011 12:09 PM, Gleb Natapov wrote:
> >>
> >>  Actually, I'd expect most read/writes to benefit from caching, no?
> >>  So why don't we just rename kvm_write_guest_cached() to
> >>  kvm_write_guest(), and the few places - if any - that need to force
> >>  transversing of the gfn mappings, get renamed to
> >>  kvm_write_guest_uncached ?
> >>
> >Good idea. I do not see any places where kvm_write_guest_uncached is
> >needed from a brief look. Avi?
> >
> 
> kvm_write_guest_cached() needs something to supply the cache, and
> needs recurring writes to the same location.  Neither of these are
> common (for example, instruction emulation doesn't have either).
> 
Correct. Missed that. So what about changing steal time to use
kvm_write_guest_cached()?

> >>
> >>  If done like you said, time spent on the hypervisor is accounted as
> >>  steal time. I don't think it is.
> >I thought that this is the point of a steal time. Running other
> >tasks/guests is a hypervisor overhead too after all :) Also what about
> >time spend serving host interrupts between put/get? It will not be
> >accounted as steal time, correct?
> 
> With accurate interrupt time accounting, it should be.  Otherwise
> general hypervisor overhead is not steal time.
> 
> (i.e. if the host is not overcommitted, steal time should be close to zero).
> 

--
			Gleb.
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