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Re: [Xen-devel] [PATCH v4] x86/arm/mm: use gfn instead of pfn in p2m_get_mem_access/p2m_set_mem_access

On 02/07/15 12:25, Tim Deegan wrote:
> At 12:09 +0100 on 02 Jul (1435838956), Andrew Cooper wrote:
>> On 02/07/15 11:48, George Dunlap wrote:
>>> Now in p2m_set_mem_access(), rather than just using an unsigned long in
>>> the loop iterating over gfns, you do this thing where you convert gfn_t
>>> to unsigned long, add one, and then convert it back to gfn_t again.
>>> I can't see any comments in v3 that suggest you doing that, and it seems
>>> a bit clunky.  Is that really necessary?  Wouldn't it be better to
>>> declare a local variable?
>>> I'm not strongly opinionated on this one, it just seems a bit strange.
>>> Everything else looks good, thanks.
>> Looping over {g,m,p}fn_t's is indeed awkward, as the compiler tricks for
>> typesafety don't allow for simply adding 1 to a typesafe variable.
>> In a cases like this, I think it is acceptable to keep a unsigned long
>> shadow variable and manipulate it is a plain integer.  The eventual
>> _gfn() required to pass it further down the callchain will help to
>> visually re-enforce the appropriate type.
>> After all, the entire point of these typesafes are to try and avoid
>> accidentally mixing up the different address spaces, but a function
>> which takes a typesafe, loops over a subset and passes the same typesafe
>> further down can probably be trusted to DTRT, catching errors at review
>> time. 
>> Off the top of my head, the only functions which would normally expect
>> to mix and match the typesafes are the pagetable walking ones.
> It should be easy enough to extend the macros to define a
> gfn_inc(&gfn_t) operator for this kind of thing.

Very true.  I suspect inc and dec will cover the majority of the
unbox/modify/box bits we currently have.

OTOH, ideally I would prefer not to introduce even more magic which
cscope/tags can't spot, but introducing gfn_inc() does appear to be the
neater solution.


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