linux-kernel - Re: [Xen-devel] [PATCH 5/7] xen/p2m: Add logic to revector a P2M tree to use _

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Message-ID: <20120727173459.GB17427@andromeda.dapyr.net>
Date:	Fri, 27 Jul 2012 13:34:59 -0400
From:	Konrad Rzeszutek Wilk <konrad@...nok.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH 5/7] xen/p2m: Add logic to revector a P2M tree to use __va leafs.

On Fri, Jul 27, 2012 at 12:47:47PM +0100, Jan Beulich wrote:
> >>> On 27.07.12 at 13:18, Stefano Stabellini <stefano.stabellini@...citrix.com> wrote:
> > On Thu, 26 Jul 2012, Konrad Rzeszutek Wilk wrote:
> >>  1) All P2M lookups instead of using the __ka address would
> >>     use the __va address. This means we can safely erase from
> >>     __ka space the PMD pointers that point to the PFNs for
> >>     P2M array and be OK.
> >>  2). Allocate a new array, copy the existing P2M into it,
> >>     revector the P2M tree to use that, and return the old
> >>     P2M to the memory allocate. This has the advantage that
> >>     it sets the stage for using XEN_ELF_NOTE_INIT_P2M
> >>     feature. That feature allows us to set the exact virtual
> >>     address space we want for the P2M - and allows us to
> >>     boot as initial domain on large machines.
> >> 
> >> So we pick option 2).
> > 
> > 1) looks like a decent option that requires less code.
> > Is the problem with 1) that we might want to access the P2M before we
> > have __va addresses ready?
> 
> AIUI 1) has no easy way of subsequently accommodating support
> for XEN_ELF_NOTE_INIT_P2M (where you almost definitely will
> want/need to reclaim the originally used VA space - if nothing else,
> then for forward compatibility with the rest of the kernel).

<nods> That was my thinking - this way we can boot dom0 (since
the hypervisor is the only one that implements the
XEN_ELF_NOTE_INIT_P2M) with large amount of memory. Granted booting
with more than 500GB would require adding another layer to the P2M
tree. But somehow I thought that we are limited in the hypervisor
to 500GB?
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