Re: [PATCH][2.6] Dont use cpu_has_pse for WP test branch

[Zwane Mwaikambo]

On Tue, 4 Nov 2003, Linus Torvalds wrote:

> Why?
> 
> The reason we test the PSE bit is not that we think it's a good indicator 
> of "new enough".  It's because if the PSE bit is set, we will use 4MB 
> pages, and the code below that actually _tests_ whether WP works or not 
> won't work.

Agreed, i also retracted the patch due to the reasons behind cpu_has_pse 
not working was because of CONFIG_DEBUG_PAGEALLOC.

> In fact, these days we could remove the test entirely: the only reason it 
> exists is because traditionally we didn't have the "fixmap" helpers, so we 
> used the page in lowest kernel memory for testing (which did not exist if 
> we had PSE, since with PSE the kernel wouldn't use individual pages to map 
> itself).

Wasn't the test unconditional in 2.4? How about the following then?

Index: linux-2.6.0-test9-mm1/arch/i386/mm/init.c
===================================================================
RCS file: /build/cvsroot/linux-2.6.0-test9-mm1/arch/i386/mm/init.c,v
retrieving revision 1.1.1.1
diff -u -p -B -r1.1.1.1 init.c
--- linux-2.6.0-test9-mm1/arch/i386/mm/init.c	30 Oct 2003 11:22:42 -0000	1.1.1.1
+++ linux-2.6.0-test9-mm1/arch/i386/mm/init.c	4 Nov 2003 16:34:45 -0000
@@ -390,12 +390,6 @@ void __init paging_init(void)
 
 void __init test_wp_bit(void)
 {
-	if (cpu_has_pse) {
-		/* Ok, all PSE-capable CPUs are definitely handling the WP bit right. */
-		boot_cpu_data.wp_works_ok = 1;
-		return;
-	}
-
 	printk("Checking if this processor honours the WP bit even in supervisor mode... ");
 
 	/* Any page-aligned address will do, the test is non-destructive */
-
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