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From: Wei Liu <wei.liu2@citrix.com>
To: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	xen-devel@lists.xen.org, Jan Beulich <jbeulich@suse.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH v4 08/16] xen/mm: Drop the parameter mfn from populate_pt_range
Date: Thu, 22 Feb 2018 16:51:24 +0000	[thread overview]
Message-ID: <20180222165124.zaufci7dew2dp23n@citrix.com> (raw)
In-Reply-To: <445bc932-044d-eacf-a487-0410e8fdb22d@arm.com>

On Thu, Feb 22, 2018 at 04:40:04PM +0000, Julien Grall wrote:
> Hi Wei,
> 
> On 22/02/18 16:35, Wei Liu wrote:
> > On Wed, Feb 21, 2018 at 02:02:51PM +0000, Julien Grall wrote:
> > > The function populate_pt_range is used to populate in advance the
> > > page-table but it will not do the actual mapping. So passing the MFN in
> > > parameter is pointless. Note that the only caller pass 0...
> > > 
> > > At the same time replace 0 by INVALID_MFN to make clear the MFN is
> > > invalid.
> > > 
> > 
> > The mfn parameter is the first mfn of a consecutive nr MFNs passed to
> > map_pages_to_xen. Putting INVALID_MFN isn't helping -- the value written
> > to page table(s) will wrap around to 0.
> > 
> > And I think starting from 0 to avoid overflow is probably a better
> > behaviour. If you really want to make sure all entries are filled with
> > INVALID_MFN you should call map_pages_to_xen for nr times with each
> > page.
> 
> I am not sure to understand this. From its name, populate_pt_range should
> only create the intermediate tables. The leaf entry will stay invalid. So
> how the value of mfn matters? Is it because the code is written in a such
> way that passing INVALID_MFN will result to undefined behavior?

Right, that's what I meant. It doesn't matter whether you use 0 or
INVALID_MFN.

Unsigned integer overflow is not UB in C, so passing INVALID_MFN is
safe.

But your intention seemed to be filling all entries with INVALID_MFN to
aid debugging, so the function doesn't do what I think you wanted it to
do. It could be I misunderstood your intention.

Wei.

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  reply	other threads:[~2018-02-22 16:51 UTC|newest]

Thread overview: 76+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-21 14:02 [PATCH v4 00/16] xen: Convert page_to_mfn and mfn_to_page to use typesafe MFN Julien Grall
2018-02-21 14:02 ` [PATCH v4 01/16] xen/tmem: Convert the file common/tmem_xen.c " Julien Grall
2018-02-21 14:02 ` [PATCH v4 02/16] xen/arm: setup: use maddr_to_mfn rather than _mfn(paddr_to_pfn(...)) Julien Grall
2018-02-21 14:02 ` [PATCH v4 03/16] xen/arm: mm: Use gaddr_to_gfn rather than _gfn(paddr_to_pfn(...)) Julien Grall
2018-02-21 14:02 ` [PATCH v4 04/16] xen/arm: mm: Remove unused M2P code Julien Grall
2018-02-21 14:02 ` [PATCH v4 05/16] xen/arm: mm: Remove unused relinquish_shared_pages Julien Grall
2018-02-21 14:02 ` [PATCH v4 06/16] xen/x86: Remove unused override of page_to_mfn/mfn_to_page Julien Grall
2018-03-01 11:20   ` George Dunlap
2018-03-02 14:42   ` Jan Beulich
2018-03-02 14:44     ` Julien Grall
2018-03-02 15:11       ` Jan Beulich
2018-03-05 13:29         ` Julien Grall
2018-02-21 14:02 ` [PATCH v4 07/16] xen/x86: mm: Switch x86/mm.c to use typesafe for virt_to_mfn Julien Grall
2018-03-02 14:45   ` Jan Beulich
2018-03-02 14:46     ` Julien Grall
2018-02-21 14:02 ` [PATCH v4 08/16] xen/mm: Drop the parameter mfn from populate_pt_range Julien Grall
2018-02-22 16:35   ` Wei Liu
2018-02-22 16:40     ` Julien Grall
2018-02-22 16:51       ` Wei Liu [this message]
2018-02-22 16:55         ` Julien Grall
2018-02-22 17:10           ` Wei Liu
2018-03-02 14:55           ` Jan Beulich
2018-03-05 13:43             ` Julien Grall
2018-03-05 14:00               ` Jan Beulich
2018-03-05 14:11                 ` Julien Grall
2018-03-05 14:38                   ` Jan Beulich
2018-03-09 17:29                     ` Wei Liu
2018-03-11 19:30                       ` Julien Grall
2018-03-12  6:36                       ` Jan Beulich
2018-03-14 15:22                         ` Julien Grall
2018-02-21 14:02 ` [PATCH v4 09/16] xen/pdx: Introduce helper to convert MFN <-> PDX Julien Grall
2018-02-22 16:39   ` Wei Liu
2018-02-21 14:02 ` [PATCH v4 10/16] xen/mm: Switch map_pages_to_xen to use MFN typesafe Julien Grall
2018-02-23  4:59   ` Tian, Kevin
2018-02-23 17:21   ` Wei Liu
2018-03-02 15:06   ` Jan Beulich
2018-03-02 15:08     ` Jan Beulich
2018-03-05 14:07     ` Julien Grall
2018-03-05 14:39       ` Jan Beulich
2018-03-05 14:44         ` Julien Grall
2018-02-21 14:02 ` [PATCH v4 11/16] xen/mm: Switch page_alloc.c to typesafe MFN Julien Grall
2018-02-23 17:21   ` Wei Liu
2018-03-02 15:18   ` Jan Beulich
2018-03-02 15:57     ` Julien Grall
2018-02-21 14:02 ` [PATCH v4 12/16] xen/mm: Switch common/memory.c to use " Julien Grall
2018-02-23 17:26   ` Wei Liu
2018-02-23 17:46     ` Julien Grall
2018-02-23 18:05       ` Wei Liu
2018-02-23 18:06         ` Julien Grall
2018-02-23 18:10           ` Wei Liu
2018-03-02 15:34   ` Jan Beulich
2018-03-05 14:18     ` Julien Grall
2018-03-05 14:41       ` Jan Beulich
2018-03-09 17:33         ` Wei Liu
2018-03-11 19:44           ` Julien Grall
2018-03-12  6:39             ` Jan Beulich
2018-03-14 16:08               ` Julien Grall
2018-02-21 14:02 ` [PATCH v4 13/16] xen/grant: Switch {create, replace}_grant_p2m_mapping to " Julien Grall
2018-02-23 17:29   ` Wei Liu
2018-03-02 15:38   ` Jan Beulich
2018-02-21 14:02 ` [PATCH v4 14/16] xen/grant: Switch common/grant_table.c to use " Julien Grall
2018-02-23 17:30   ` Wei Liu
2018-03-02 15:54   ` Jan Beulich
2018-03-02 15:59     ` Julien Grall
2018-03-02 16:12       ` Jan Beulich
2018-02-21 14:02 ` [PATCH v4 15/16] xen/x86: Switch mfn_to_page in x86_64/mm.c " Julien Grall
2018-03-02 15:57   ` Jan Beulich
2018-02-21 14:02 ` [PATCH v4 16/16] xen: Convert page_to_mfn and mfn_to_page " Julien Grall
2018-02-21 14:25   ` Razvan Cojocaru
2018-02-21 14:59   ` Paul Durrant
2018-02-21 23:20   ` Boris Ostrovsky
2018-02-23  4:59   ` Tian, Kevin
2018-02-23 17:31   ` Wei Liu
2018-03-02 16:08   ` Jan Beulich
2018-03-14 17:02     ` Julien Grall
2018-03-15  7:07       ` Jan Beulich

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