Re: [PATCH v2 03/13] iommu: make use of type-safe BFN and MFN in exported functions

From: "Jan Beulich" <JBeulich@suse.com>
To: Paul Durrant <paul.durrant@citrix.com>
Cc: Kevin Tian <kevin.tian@intel.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Tim Deegan <tim@xen.org>,
	george.dunlap@citrix.com, Julien Grall <julien.grall@arm.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	IanJackson <Ian.Jackson@citrix.com>
Subject: Re: [PATCH v2 03/13] iommu: make use of type-safe BFN and MFN in exported functions
Date: Tue, 10 Jul 2018 08:28:49 -0600	[thread overview]
Message-ID: <5B44C2A102000078001D2BF9@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <985628ec89744222bac572772231bed8@AMSPEX02CL03.citrite.net>

>>> On 10.07.18 at 16:10, <Paul.Durrant@citrix.com> wrote:
>>  -----Original Message-----
>> From: George Dunlap [mailto:george.dunlap@citrix.com]
>> Sent: 10 July 2018 15:01
>> To: Paul Durrant <Paul.Durrant@citrix.com>; xen-devel@lists.xenproject.org 
>> Cc: Jan Beulich <jbeulich@suse.com>; Andrew Cooper
>> <Andrew.Cooper3@citrix.com>; George Dunlap
>> <George.Dunlap@citrix.com>; Ian Jackson <Ian.Jackson@citrix.com>; Konrad
>> Rzeszutek Wilk <konrad.wilk@oracle.com>; Stefano Stabellini
>> <sstabellini@kernel.org>; Julien Grall <julien.grall@arm.com>; Tim (Xen.org)
>> <tim@xen.org>; Wei Liu <wei.liu2@citrix.com>; Jun Nakajima
>> <jun.nakajima@intel.com>; Kevin Tian <kevin.tian@intel.com>
>> Subject: Re: [PATCH v2 03/13] iommu: make use of type-safe BFN and MFN
>> in exported functions
>> 
>> On 07/07/2018 12:05 PM, Paul Durrant wrote:
>> > This patch modifies the declaration of the entry points to the IOMMU
>> > sub-system to use bfn_t and mfn_t in place of unsigned long. A
>> subsequent
>> > patch will similarly modify the methods in the iommu_ops structure.
>> >
>> > Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
>> > ---
>> > Cc: Jan Beulich <jbeulich@suse.com>
>> > Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>> > Cc: George Dunlap <George.Dunlap@eu.citrix.com>
>> > Cc: Ian Jackson <ian.jackson@eu.citrix.com>
>> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> > Cc: Stefano Stabellini <sstabellini@kernel.org>
>> > Cc: Julien Grall <julien.grall@arm.com>
>> > Cc: Tim Deegan <tim@xen.org>
>> > Cc: Wei Liu <wei.liu2@citrix.com>
>> > Cc: Jun Nakajima <jun.nakajima@intel.com>
>> > Cc: Kevin Tian <kevin.tian@intel.com>
>> > Cc: George Dunlap <george.dunlap@eu.citrix.com>
>> >
>> > v2:
>> >  - Addressed comments from Jan.
>> >  - Use intermediate 'frame' variable to avoid directly encapsulating
>> >    mfn or gfn values as bfns.
>> 
>> Explain this one to me?  At the moment I don't see any value from having
>> the extra variable in the middle.
> 
> This was something that Jan wanted.

Ah, yes, it was in a reply to this patch's v1 that I had suggested a
neutrally named variable in case it can hold values from more than
one space. In the particular case of _get_page_type(), where I had
also given the v1 comment, I can't however see that it is now really
obvious that a 1:1 mapping is being established. To me that would
mean passing the same local variable (suitably type wrapped where
needed) twice into iommu_map_page(). It is not clear to me what
use a mfn_to_gmfn() invocation is inside a is_pv_domain() guarded
block, now that we don't permit translated PV domains anymore
(not that I would think that they had worked before the code was
removed).

Jan

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