Re: issue with patch "x86: no CPA on iounmap"

[Arjan van de Ven <arjan@linux.intel.com>]

Ingo Molnar wrote:
> * Arjan van de Ven <arjan@linux.intel.com> wrote:
> 
>> Siddha, Suresh B wrote:
>>> This is wrt to x86 git commit f56d005d30342a45d8af2b75ecccc82200f09600
>>> 	"x86: no CPA on iounmap"
>>>
>>> This can use performance issue. When a GART driver unmaps a RAM page,
>> thinking about this some more...
>>
>> afaik the gart driver doesn't use ioremap....
>>
>> (and it does caching control explicitly, and sets its pages back to 
>> cached)
> 
> there are many GART drivers, and the method used depends on the GART 
> driver. The following GART drivers still use ioremap in one way or 
> another:
> 
>  drivers/char/agp/amd-k7-agp.c
>  drivers/char/agp/ati-agp.c
>  drivers/char/agp/generic.c
>  drivers/char/agp/sworks-agp.c
>  drivers/char/drm/radeon_cp.c
> 
> the method use is in all cases the same: they use __get_free_page() to 
> pick up a general RAM page, they do SetPageReserved() and then they use 
> ioremap_nocache() to map it non-cached, and then they also program the 
> GART to access those pages.
> 
> when the GART code deinits, it does an iounmap() on those pages, unmaps 
> it from the GART hardware itself, does a ClearPageReserved() and does 
> __free_page() to put the page into the general page pool again. So 
> Suresh is right: these pages are currently marked UC at this point and 
> we need to mark them cacheable.
> 
> we could do this automatically in iounmap() upon seeing a page_is_ram() 
> that has PageReserved set. Or we could stick in a set_memory_wb() into 
> the deinit [and ioremap_nocache()-failure] sequence.
> 
> Since we treat PageReserved pages specially in ioremap() already [we 
> allow them, despite them being listed in the e820 map], i think the more 
> robust solution is to recognize them in iounmap() as well - this way it 
> cannot be forgotten accidentally. (and UC pages in the buddy are _hard_ 
> to notice after the fact) There is no aliasing danger i believe: IO bars 
> should never be marked as general RAM in the e820.
> 

agreed, esp for .25

it's sort of a weird case of ioremap() use; I wonder if longer term we need
to have a different sort of interface for this kind of use...