Re: [patch] x86, pat: allow ISA memory range uncacheable mapping requests

["H. Peter Anvin" <hpa@zytor.com>]

On 08/17/2009 01:23 PM, Suresh Siddha wrote:
> Max Vozeler reported:
>>  Bug 13877 -  bogl-term broken with CONFIG_X86_PAT=y, works with =n  
>>
>>  strace of bogl-term:
>>  814   mmap2(NULL, 65536, PROT_READ|PROT_WRITE, MAP_SHARED, 4, 0)
>> 				 = -1 EAGAIN (Resource temporarily unavailable)
>>  814   write(2, "bogl: mmaping /dev/fb0: Resource temporarily unavailable\n",
>> 	       57) = 57
> 
> PAT code maps the ISA memory range as WB in the PAT attribute, so that
> fixed range MTRR registers define the actual memory type (UC/WC/WT etc).
> 
> But the upper level is_new_memtype_allowed() API checks are failing,
> as the request here is for UC and the return tracked type is WB (Tracked type is
> WB as MTRR type for this legacy range potentially will be different for each
> 4k page).
> 
> Fix is_new_memtype_allowed() by always succeeding the ISA address range
> checks, as the null PAT (WB) and def MTRR fixed range register settings
> satisfy the memory type needs of the applications that map the ISA address
> range.

This patch seems correct in that it matches the current behavior of the
code.  I have, though, to ask what the logic behind treating the ISA
region in this way is.  From a hardware perspective it makes sense --
these addresses have the Legacy MTRRs which are like a
physical-address-based PAT, but it seems somewhat odd that'd we would
expect applications to use different APIs for this region.

I think the patch is definitely OK for x86/urgent, but I'd like some
thoughts about if this really is The Right Thing in the long term?

	-hpa