Subject: RE: [PATCH 2/2] powerpc: Make context bits depend on virtual addr size.

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 13 Feb 2013 19:56:44 +0530

David Laight <[email-masked]> writes:

>> +#define CONTEXT_BITS		19
>> +#define USER_ESID_BITS		18
>> +#define USER_ESID_BITS_1T	6
>> +
>> +/*
>> + * 256MB segment
>> + * The proto-VSID space has 2^(CONTEX_BITS + USER_ESID_BITS) - 1 segments
>> + * available for user + kernel mapping. The top 4 contexts are used for
>> + * kernel mapping. Each segment contains 2^28 bytes. Each
>> + * context maps 2^46 bytes (64TB) so we can support 2^19-1 contexts
>> + * (19 == 37 + 28 - 46).
>> + */
>
> I can't help feeling this would be easier to understand if a full
> 64? 80? bit address was shown with the various bit ranges identified.
>
> Given the comment, I'd have expected CONTEXT_BITS to be calculated
> from three other named constants - rather than being set to 19.
>

May be the comments were misleading. We build proto vsid using a
combination of context and ea bits.

Current code does the below:

for kernel:
proto_vsid = ea >> SID_SHIFT;
proto_vsid |= (1UL << (CONTEXT_BITS + USER_ESID_BITS));
for user:
proto_vsid = ea >> SID_SHIFT
proto_vsid |= context << USER_ESID_BITS 

context range is 0 - (2^19 -1)

With this patch we _don't_ give kernel half the proto vsid range.
Instead, we reduce the proto vsid range and then the kernel is given
top 4 context. ie, kernel proto vsid is now

for kenel:
proto_vsid = ea >> SID_SHIFT;
context = (MAX_CONTEXT - 4) +  ((effective address >> 60) - 0xc);
proto_vsid |= context << USER_ESID_BITS 

-aneesh