Re: [PATCH] x86, AMD: Correct F15h IC aliasing issue

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

On 07/26/2011 12:42 PM, Andre Przywara wrote:
>>
>> In other words, it's completely ad hoc.
> 
> There is no need to determine it by calculating, because it caused by 
> the special design of the BD L1 cache and thus fixed.
> And a calculation would be even more confusing:
> 
> The L1I is virtually indexed, but physically tagged.
> 64KB L1I cache, 64 Bytes per Cacheline = 1024 cache lines
> 1024 lines / 2 way associative = 512 indexes
> 64 Bytes per Cacheline (6 bits) + 512 indexes (9 bits) = bits [14:0]
> virtual and physical addresses are the same for bits [11:0], which 
> leaves the remaining 14:12 susceptible for aliasing.
> 
> So bit 12 comes from PAGESIZE and yes, the 14 could be derived from the 
> CPUID cache info, but I don't see much value in breaking this down this way.
> But I agree that there should be some comment in the patch which at 
> least notes that bits [14:12] are due to the L1I design, maybe we can 
> copy a nicer version of the above math in the commit message for reference.
> 

The right thing to do this would be to have a bit in CPUID which would
indicate that the kernel has to perform the above calculation -- because
obviously not all CPUs have this issue.  From that we can calculate a
mask to insert into whatever appropriate location... depending on the
exact tack we have to apply to deal with this situation.

	-hpa