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


  • To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Tue, 10 Jul 2018 17:19:34 +0100
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  • Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, "Tim \(Xen.org\)" <tim@xxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxx>
  • Delivery-date: Tue, 10 Jul 2018 16:19:40 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 07/10/2018 05:18 PM, Paul Durrant wrote:
>> -----Original Message-----
>> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
>> Sent: 10 July 2018 17:13
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>; Andrew Cooper
>> <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap
>> <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Konrad
>> Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini
>> <sstabellini@xxxxxxxxxx>; Julien Grall <julien.grall@xxxxxxx>; Tim (Xen.org)
>> <tim@xxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Jun Nakajima
>> <jun.nakajima@xxxxxxxxx>; Kevin Tian <kevin.tian@xxxxxxxxx>
>> 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@xxxxxxxxxx>
>>> ---
>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
>>> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>> Cc: Julien Grall <julien.grall@xxxxxxx>
>>> Cc: Tim Deegan <tim@xxxxxxx>
>>> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
>>> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
>>> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
>>> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>>>
>>> v2:
>>>  - Addressed comments from Jan.
>>>  - Use intermediate 'frame' variable to avoid directly encapsulating
>>>    mfn or gfn values as bfns.
>>
>> A couple of comments on particular instances...
>>
>>> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
>>> index c53cab44d9..ce12bcff42 100644
>>> --- a/xen/arch/x86/mm/p2m.c
>>> +++ b/xen/arch/x86/mm/p2m.c
>>> @@ -714,9 +714,12 @@ p2m_remove_page(struct p2m_domain *p2m,
>> unsigned long gfn_l, unsigned long mfn,
>>>
>>>          if ( need_iommu(p2m->domain) )
>>>          {
>>> +            unsigned long frame = mfn;
>>> +            bfn_t bfn = _bfn(frame);
>>> +
>>>              for ( i = 0; i < (1 << page_order); i++ )
>>>              {
>>> -                int ret = iommu_unmap_page(p2m->domain, mfn + i);
>>> +                int ret = iommu_unmap_page(p2m->domain, bfn_add(bfn, i));
>>
>> Having a 'bfn' variable here makes some sense, because otherwise, if mfn
>> ever gets the mfn_t type, you'll have
>>
>>   iommu_unmap_page(... _bfn(mfn_x(mfn)+i));
>>
>> being able to use bfn_add() is much cleaner.  I don't think the
>> intermediate 'frame' variable in the case, really adds anything.
>>
>>> diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
>>> index d2610e320c..d0926d13e0 100644
>>> --- a/xen/common/grant_table.c
>>> +++ b/xen/common/grant_table.c
>>> @@ -1132,6 +1132,8 @@ map_grant_ref(
>>>      need_iommu = gnttab_need_iommu_mapping(ld);
>>>      if ( need_iommu )
>>>      {
>>> +        unsigned long frame = mfn_x(mfn);
>>> +        bfn_t bfn = _bfn(frame);
>>>          unsigned int kind;
>>>          int err = 0;
>>>
>>> @@ -1144,14 +1146,13 @@ map_grant_ref(
>>>               !(old_pin & (GNTPIN_hstw_mask|GNTPIN_devw_mask)) )
>>>          {
>>>              if ( !(kind & MAPKIND_WRITE) )
>>> -                err = iommu_map_page(ld, mfn_x(mfn), mfn_x(mfn),
>>> -                                     IOMMUF_readable|IOMMUF_writable);
>>> +                err = iommu_map_page(ld, bfn, mfn,
>>> +                                     IOMMUF_readable | IOMMUF_writable);
>>>          }
>>>          else if ( act_pin && !old_pin )
>>>          {
>>>              if ( !kind )
>>> -                err = iommu_map_page(ld, mfn_x(mfn), mfn_x(mfn),
>>> -                                     IOMMUF_readable);
>>> +                err = iommu_map_page(ld, bfn, mfn, IOMMUF_readable);
>>
>> Here's an example where I think having an extra variable is somewhat
>> dangerous.  Before this change, it's obvious that you have a 1:1
>> mapping; now, looking just at this line, it's not obvious that bfn ==
>> mfn.  Worse, there's a risk that there will be some sort of bug
>> introduced which changes bfn, such that bfn != mfn anymore.
>>
>> If you have to use an intermediate variable here, this should be
>>
>>   iommu_map_page(..., _bfn(frame), _mfn(frame), ...);
>>
>> But I really think
>>
>>   iommu_map_page(..., _bfn(mfn_x(mfn)), mfn, ...);
>>
>> makes the most sense here.
> 
> How about:
> 
> #define mfn_to_bfn(mfn) (_bfn(mfn_x(mfn))
> 
> iommu_map_page(..., mfn_to_bfn(mfn), mfn, ...);
> 
> ?
> 
> I can similarly define gfn_to_bfn() for places where it is needed.

That works for me.

 -George

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