linux-kernel - Re: [PATCH 3/8] xen/setup: Set identity mapping for non-RAM E820 and E820 gaps.

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Message-ID: <20110104183822.GA1505@dumpdata.com>
Date:	Tue, 4 Jan 2011 13:38:22 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"hpa@...or.com" <hpa@...or.com>, Jan Beulich <JBeulich@...ell.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>
Subject: Re: [PATCH 3/8] xen/setup: Set identity mapping for non-RAM E820
 and E820 gaps.

On Tue, Jan 04, 2011 at 05:18:58PM +0000, Ian Campbell wrote:
> On Thu, 2010-12-30 at 19:48 +0000, Konrad Rzeszutek Wilk wrote:
> > We walk the E820 region and start at 0 (for PV guests we start
> > at ISA_END_ADDRESS)
> 
> I was trying to figure out what any of this had to do with HVM guests,
> but you mean as opposed to dom0, which with my pedant hat on is also a
> guest ;-).
> 
> >  and skip any E820 RAM regions. For all other
> > regions and as well the gaps we set them to be identity mappings.
> > 
> > The reasons we do not want to set the identity mapping from 0->
> > ISA_END_ADDRESS when running as PV is b/c that the kernel would
> > try to read DMI information and fail (no permissions to read that).
> 
> The reason for this special case is that in domU we have already punched
> a hole from 640k-1M into the e820 which the hypervisor gave us.

For the privileged guest - yes. But for the non-priviligied it does not have
such range and would end up failing.
> 
> Should we perhaps be doing this identity mapping before we punch that
> extra hole? i.e. setup ID mappings based on the hypervisors idea of the
> guest e820 not the munged one we subsequently magicked up? Only the

You mean the ISA_START_ADDRESS->ISA_END_ADDRESS we mark as reserved?

It sure would be easier (and it would mean we can return that memory
back to the hypervisor).

> original e820 is going to bear any possible resemblance to the identity
> pages which the guest can actually see.
> 
> Ian.
> 
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