linux-kernel - Re: [Xen-devel] [PATCH 08/11] xen p2m: transparently change the p2m mappings in the m2p override

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Date:	Thu, 16 Dec 2010 15:25:14 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [Xen-devel] [PATCH 08/11] xen p2m: transparently change the p2m
 mappings in the m2p override

On Wed, 15 Dec 2010, Jeremy Fitzhardinge wrote:
> On 12/15/2010 05:40 AM, stefano.stabellini@...citrix.com wrote:
> > From: Stefano Stabellini <stefano.stabellini@...citrix.com>
> >
> > In m2p_add_override store the original mfn into page->index and then
> > change the p2m mapping, setting mfns as FOREIGN_FRAME.
> >
> > In m2p_remove_override restore the original mapping.
> >
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > ---
> >  arch/x86/xen/p2m.c |    4 ++++
> >  1 files changed, 4 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > index 8db19d5..7dde8e4 100644
> > --- a/arch/x86/xen/p2m.c
> > +++ b/arch/x86/xen/p2m.c
> > @@ -407,8 +407,11 @@ static unsigned long mfn_hash(unsigned long mfn)
> >  void m2p_add_override(unsigned long mfn, struct page *page)
> >  {
> >  	unsigned long flags;
> > +	unsigned long pfn = page_to_pfn(page);
> >  	page->private = mfn;
> > +	page->index = pfn_to_mfn(pfn);
> 
> Is there any risk that a page being used for a granted mfn will end up
> getting xen_create_contiguous_region() applied to it via the DMA API? 
> That would be messy...

I don't think so because AFAIK xen_create_contiguous_region is only
called:

- by xen_swiotlb_fixup on xen_io_tlb_start (+ offset) that has been
allocated using alloc_bootmem;

- by xen_swiotlb_alloc_coherent on memory allocated using
  __get_free_pages.

if in the future xen_create_contiguous_region will be called on other
memory ranges then maybe, but at the moment there are no problems.
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