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parisc: shmparam.h: Document aliasing requirements of PA-RISC

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Add some documentation why PA-RISC uses SHMLBA and SHM_COLOUR.

Signed-off-by: Helge Deller <deller@gmx.de>
This commit is contained in:
Helge Deller
2023-09-01 08:04:31 +02:00
parent b1bef1388c
commit 3f091387a3
1 changed files with 15 additions and 0 deletions
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arch/parisc/include/asm/shmparam.h
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@@ -2,6 +2,21 @@
#ifndef _ASMPARISC_SHMPARAM_H
#define _ASMPARISC_SHMPARAM_H
/*
* PA-RISC uses virtually indexed & physically tagged (VIPT) caches
* which has strict requirements when two pages to the same physical
* address are accessed through different mappings. Read the section
* "Address Aliasing" in the arch docs for more detail:
* PA-RISC 1.1 (page 3-6):
* https://parisc.wiki.kernel.org/images-parisc/6/68/Pa11_acd.pdf
* PA-RISC 2.0 (page F-5):
* https://parisc.wiki.kernel.org/images-parisc/7/73/Parisc2.0.pdf
*
* For Linux we allow kernel and userspace to map pages on page size
* granularity (SHMLBA) but have to ensure that, if two pages are
* mapped to the same physical address, the virtual and physical
* addresses modulo SHM_COLOUR are identical.
*/
#define SHMLBA PAGE_SIZE /* attach addr a multiple of this */
#define SHM_COLOUR 0x00400000 /* shared mappings colouring */