linux-kernel - Re: [PATCH][v2.6.29][XEN] Return unused memory to hypervisor

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Message-ID: <19086.35370.70561.378736@gianluca.uk.xensource.com>
Date:	Fri, 21 Aug 2009 12:51:06 +0100
From:	Gianluca Guida <gianluca.guida@...rix.com>
To:	Miroslav Rezanina <mrezanin@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Gianluca Guida <Gianluca.Guida@...citrix.com>
Subject: Re: [PATCH][v2.6.29][XEN] Return unused memory to hypervisor

Miroslav Rezanina writes:
 > ----- Original Message -----
 > From: "Jeremy Fitzhardinge" <jeremy@...p.org>
 > To: "Miroslav Rezanina" <mrezanin@...hat.com>
 > Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com, "Gianluca Guida" <gianluca.guida@...rix.com>
 > Sent: Thursday, August 20, 2009 6:39:02 PM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna
 > Subject: Re: [PATCH][v2.6.29][XEN] Return unused memory to hypervisor
 > 
 > On 08/20/09 00:47, Miroslav Rezanina wrote:
 > >> there is handled e820 map in guest. However, this patch informs 
 > >> hypervisor, that guest uses less memory than was assigned to it. 
 > >> If hypervisor is not informed, memory is reserved for guest that
 > >> do not need it. If hypervisor is informed, he decrease memory
 > >> reservation for guest and unused memory is marked as free 
 > >> for use by other guests.
 > >>   
 > >
 > > Yes.  But the guest will modify its own e820 map for a number of
 > > reasons; for example: reducing its own memory, or clearing a space for
 > > the PCI hole.  In general we want to free any underlying pages which
 > > don't correspond to E820_RAM regions.
 > >
 > >    J
 > 
 > I agree. However, I'm not sure if xen supports such a precise handling.

Xen will just get a list of mfn to remove from the domain. That is
precise enough. The scenario Jeremy is describing does happen,
especially in dom0 kernels.

G.
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