WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

[Xen-devel] Re: Xen MMU's requirement to pin pages RO and initial_memory

To: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Subject: [Xen-devel] Re: Xen MMU's requirement to pin pages RO and initial_memory_mapping.
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Mon, 23 May 2011 16:20:15 +0100
Cc: "jeremy@xxxxxxxx" <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, "hpa@xxxxxxxxx" <hpa@xxxxxxxxx>, "hpa@xxxxxxxxxxxxxxx" <hpa@xxxxxxxxxxxxxxx>, "yinghai@xxxxxxxxxx" <yinghai@xxxxxxxxxx>
Delivery-date: Mon, 23 May 2011 08:18:29 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <20110517180520.GC13706@xxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <20110513153010.GB16519@xxxxxxxxxxxx> <alpine.DEB.2.00.1105131650550.8972@kaball-desktop> <20110516154132.GA12486@xxxxxxxxxxxx> <alpine.DEB.2.00.1105171745300.12963@kaball-desktop> <20110517180520.GC13706@xxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Alpine 2.00 (DEB 1167 2008-08-23)
On Tue, 17 May 2011, Konrad Rzeszutek Wilk wrote:
> On Tue, May 17, 2011 at 06:50:55PM +0100, Stefano Stabellini wrote:
> > On Mon, 16 May 2011, Konrad Rzeszutek Wilk wrote:
> > > > They become pagetable pages when:
> > > > 
> > > > - they are explicitly pinned by pin_pagetable_pfn
> > > > 
> > > > - they are hooked into the current pagetable
> > > 
> > > Ok, so could we use those two calls to trigger the pagetable walk
> > > and mark them RO as appropiate? Which call sites are those? The
> > > xen_set_pgd/xen_set_pud/xen_set_pmd ?
> > 
> > xen_alloc_pte_init and xen_alloc_pmd_init are the ones that mark the
> > pagetable pages RO and pin them, calling make_lowmem_page_readonly and
> > pin_pagetable_pfn.
> > 
> > alloc_pte/pmd are called right before hooking them into the pagetable;
> > unfortunately that means that they fail at marking the pagetable pages
> > RO: make_lowmem_page_readonly uses lookup_address to find the pte
> > corresponding to a page, however at this point the pagetable pages are
> > not mapped yet (usually they are not hooked but when they are hooked, the
> > upper level pagetable page is not hooked), so lookup_address fails.
> 
> Right. We don't have to walk the hooked pagetable, I think. We are passed
> in the PMD/PGD of the PFN and we could look at the content of that PFN.
> Walk each entry in there and for those that are present, determine
> if the page table it points to (whatever level it is) is RO. If not, mark
> it RO. And naturally do it recursively to cover all levels.
 
I am not sure what you mean.
The interface is the following:

void alloc_pte(struct mm_struct *mm, unsigned long pfn);

pfn is the pagetable page's pfn, that has to be marked RO in all his mappings;
mm is the mm_struct where this pagetable page is mapped.
Except it is not true anymore because the pagetable page is not mapped
yet in mm; so we cannot walk anything here, unfortunately.


We could remember that we failed to mark this page RO so that the next
time we try to write a pte that contains that address we know we have to
mark it RO.
But this approach is basically equivalent to the one we had before
2.6.39: we consider the range pgt_buf_start-pgt_buf_end a "published"
range of pagetable pages that we have to mark RO.
In fact it is affected by the same problem: after writing the ptes that
map the range pgt_buf_start-pgt_buf_end, if we try to allocate a new
pagetable page incrementing pgt_buf_end we fail because the new page is
already marked RW in a pinned page.
At the same time we cannot modify the pte to change the mapping to RO
because lookup_address doesn't find it (the pagetable page containing
the pte in question is not reachable from init_mm yet).


Unfortunately I cannot see an easy way to fix alloc_pte without making
sure that the pfn passed as an argument is already mapped and the pte is
reachable using lookup_address.

Alternatively we could come up with a new interface that properly
publishes the pgt_buf_start-pgt_buf_top range, but it would still need a
"free" function for the pgt_buf_end-pgt_buf_top range to be called after
the initial mapping is complete.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel