svn commit: r327950 - in head/sys/powerpc: aim include powerpc ps3

[Nathan Whitehorn]

On 01/14/18 00:30, Konstantin Belousov wrote:
> On Sat, Jan 13, 2018 at 08:31:40PM -0800, Nathan Whitehorn wrote:
>>
>> On 01/13/18 15:28, Nathan Whitehorn wrote:
>>>
>>> On 01/13/18 15:24, Konstantin Belousov wrote:
>>>> On Sat, Jan 13, 2018 at 11:14:53PM +0000, Nathan Whitehorn wrote:
>>>>> +/*
>>>>> + * We (usually) have a direct map of all physical memory. All
>>>>> + * uses of this macro must be gated by a check on hw_direct_map!
>>>>> + * The location of the direct map may not be 1:1 in future, so use
>>>>> + * of the macro is recommended; it may also grow an assert that
>>>>> hw_direct_map
>>>>> + * is set.
>>>>> + */
>>>>> +#define PHYS_TO_DMAP(x) x
>>>>> +#define DMAP_TO_PHYS(x) x
>>>> Take a look at the sys/vm/vm_page.c:vm_page_free_prep() function.
>>>>
>>> I think the checks in there should work as designed, unless I'm
>>> missing something. Am I?
>>> -Nathan
>>>
>> Actually, wait, this is broken if hw_direct_map is not set. I can do an
>> #ifdef __powerpc__ hack, but do you have opinions for a better MI flag
>> for "yes, the macro is defined but, no, the direct map may not be
>> available"?
> Exactly.  You explicitly noted the need to check for the hw_direct_map
> in the comment, so I did not see a need to explain further.
>
> We intended that PHYS_TO_DMAP/DMAP_TO_PHYS become MI KPI.  If the symbols
> are present, their semantic is the unconditional presence and usability of
> the direct map.
>
> sf bufs were rototiled with things like SFBUF_OPTIONAL_DIRECT_MAP, but I
> dislike it and hope that PHYS_TO_DMAP would be not damaged this way.
>

That's unfortunate. Is there any reason you need this to be certain at 
compile time? That seems to be quite restrictive and not to add a huge 
amount of performance. Given the exciting variety of MMU modes on 
PowerPC, there is not any way to guarantee the presence of a direct map 
at compile time, so the alternative is to invent a whole new kernel 
signalling mechanism for "direct map is almost certainly available, but 
might not be", which seems strictly worse, or to have a standard API 
that PowerPC can't use, which also seems worse.
-Nathan