__read_only in the kernel

[Mateusz Guzik]

On Thu, Dec 01, 2016 at 08:05:54AM +1100, Bruce Evans wrote:
> On Wed, 30 Nov 2016, Mateusz Guzik wrote:
> >>diff --git a/sys/amd64/include/param.h b/sys/amd64/include/param.h
> >>index a619e395..ab66e79 100644
> >>--- a/sys/amd64/include/param.h
> >>+++ b/sys/amd64/include/param.h
> >>@@ -152,4 +152,6 @@
> >> #define	INKERNEL(va) (((va) >= DMAP_MIN_ADDRESS && (va) < DMAP_MAX_ADDRESS) \
> >>     || ((va) >= VM_MIN_KERNEL_ADDRESS && (va) < VM_MAX_KERNEL_ADDRESS))
> >>
> >>+#define	__read_mostly	__attribute__((__section__(".data.read_mostly")))
> >>+
> >> #endif /* !_AMD64_INCLUDE_PARAM_H_ */
> 
> 1. Hard-coded gccism: __attribute__().
> 1a.  Non-use of the FreeBSD macro __section() whose purpose is to make it
>    easy to avoid using __attribute__() for precisely what it is used for
>    here.
> 2. Misplaced definition.  Such definitions go in <sys/cdefs.h>.  This one
>    has no dependencies on amd64 except possibly for bugs elsewhere, but
>    is only in an amd64 header.
> 
[..]
> According to userland tests, section statements like the above and the
> ones in <sys/cdefs.h> don't need any linker support to work, since they
> create sections as necessary.
> 
> So the above definition in <sys/cdefs.h> should almost perfectly for
> all arches, even without linker support.  Style bug (1) is smaller if
> it is there.
> 

I wanted to avoid providing the definition for archs which don't have
the linker script bit and this was the only header I found which is md
and effectively always included.

Indeed it seems the section is harmless even without the explicit
support.

> >>diff --git a/sys/conf/ldscript.amd64 b/sys/conf/ldscript.amd64
> >>index 5d86b03..ae98447 100644
> >>--- a/sys/conf/ldscript.amd64
> >>+++ b/sys/conf/ldscript.amd64
> >>@@ -151,6 +151,11 @@ SECTIONS
> >>     KEEP (*(.gnu.linkonce.d.*personality*))
> >>   }
> >>   .data1          : { *(.data1) }
> >>+  .data_read_mostly :
> >>+  {
> >>+   *(.data.read_mostly)
> >>+   . = ALIGN(64);
> >>+  }
> >>   _edata = .; PROVIDE (edata = .);
> >>   __bss_start = .;
> >>   .bss            :
> 
> For arches without this linker support, the variables would be grouped
> but not aligned so much.
> 
> Aligning the subsection seems to be useless anyway.  This only aligns
> the first variable in the subsection.  Most variables are still only
> aligned according to their natural or specified alignment.  This is
> rarely as large as 64.  But I think variables in the subsection can
> be more aligned than the subsection.  If they had to be (as in a.out),
> then it is the responsibility of the linker to align the subsection
> to more than the default if a single variable in the subsection needs
> more than the default.
> 

With the indended use grouping side-by-side is beneficial - as the vars
in question are supposed to be rarely modified, there is no problem with
them sharing a cache line. Making them all use dedicated lines would
only waste memory.

That said, what about the patch below. I also grepped the tree and found
2 surprises, handled in the patch. 

diff --git a/sys/compat/linuxkpi/common/include/linux/compiler.h b/sys/compat/linuxkpi/common/include/linux/compiler.h
index c780abc..c32f1fa 100644
--- a/sys/compat/linuxkpi/common/include/linux/compiler.h
+++ b/sys/compat/linuxkpi/common/include/linux/compiler.h
@@ -67,7 +67,6 @@
 #define typeof(x)			__typeof(x)
 
 #define	uninitialized_var(x)		x = x
-#define	__read_mostly __attribute__((__section__(".data.read_mostly")))
 #define	__always_unused			__unused
 #define	__must_check			__result_use_check
 
diff --git a/sys/conf/ldscript.amd64 b/sys/conf/ldscript.amd64
index 5d86b03..d87d607 100644
--- a/sys/conf/ldscript.amd64
+++ b/sys/conf/ldscript.amd64
@@ -151,6 +151,11 @@ SECTIONS
     KEEP (*(.gnu.linkonce.d.*personality*))
   }
   .data1          : { *(.data1) }
+  .data.read_mostly :
+  {
+   *(.data.read_mostly)
+  }
+  . = ALIGN(64);
   _edata = .; PROVIDE (edata = .);
   __bss_start = .;
   .bss            :
diff --git a/sys/dev/drm2/drm_os_freebsd.h b/sys/dev/drm2/drm_os_freebsd.h
index dc01c6a..11c9feb 100644
--- a/sys/dev/drm2/drm_os_freebsd.h
+++ b/sys/dev/drm2/drm_os_freebsd.h
@@ -80,7 +80,6 @@ typedef void			irqreturn_t;
 
 #define	__init
 #define	__exit
-#define	__read_mostly
 
 #define	BUILD_BUG_ON(x)		CTASSERT(!(x))
 #define	BUILD_BUG_ON_NOT_POWER_OF_2(x)
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
index a1ce9b4..5f646ff 100644
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -445,4 +445,6 @@ extern void (*softdep_ast_cleanup)(void);
 
 void counted_warning(unsigned *counter, const char *msg);
 
+#define	__read_mostly		__section(".data.read_mostly")
+
 #endif /* !_SYS_SYSTM_H_ */
-- 
Mateusz Guzik <mjguzik gmail.com>