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

[Tim Deegan <tim@xxxxxxx>]

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.

Tim.

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