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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 07/02/2015 01:26 PM, Vitaly Kuznetsov wrote:
> George Dunlap <george.dunlap@xxxxxxxxxxxxx> writes:
> 
>> On 07/02/2015 12:25 PM, 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.
>>
>> I was thinking that -- although in this case you'd still need to un-pack
>> it to do the loop exit conditional.  To really make things pretty you'd
>> want a for_gfn_range() macro or something like that that takes a start
>> gfn and a number.
>>
>> But that's really starting to be feature creep for this patch, which is
>> why I didn't want to suggest it on v4. :-)
> 
> Well, if you look at what I was fixing in v1 ... :-)
> 
> I suggest we add a local unsigned long here and close the deal, when
> {g,m,p}fn_{inc,dec} macros are available we'll get rid of it.

+1

 -G


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