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

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

[deller]

From: Helge Deller <deller@gmx.de>

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

Signed-off-by: Helge Deller <deller@gmx.de>
---
 arch/parisc/include/asm/shmparam.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/parisc/include/asm/shmparam.h b/arch/parisc/include/asm/shmparam.h
index 74f74e4d35b7..5a95b0f62b87 100644
--- a/arch/parisc/include/asm/shmparam.h
+++ b/arch/parisc/include/asm/shmparam.h
@@ -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 */
 
-- 
2.41.0

Re: [PATCH] parisc: shmparam.h: Document aliasing requirements of PA-RISC

[John David Anglin]

On 2023-09-01 2:04 a.m., deller@kernel.org wrote:
> From: Helge Deller <deller@gmx.de>
>
> Add some documentation why PA-RISC uses SHMLBA and SHM_COLOUR.
>
> Signed-off-by: Helge Deller <deller@gmx.de>
> ---
>   arch/parisc/include/asm/shmparam.h | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/arch/parisc/include/asm/shmparam.h b/arch/parisc/include/asm/shmparam.h
> index 74f74e4d35b7..5a95b0f62b87 100644
> --- a/arch/parisc/include/asm/shmparam.h
> +++ b/arch/parisc/include/asm/shmparam.h
> @@ -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 */
Does SHM_COLOUR need to be exposed to user space?  See the following issue with sysprof:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1021853

Dave

-- 
John David Anglin  dave.anglin@bell.net