x86/ldt: Simplify the LDT switching logic

Originally, Linux reloaded the LDT whenever the prev mm or the next
mm had an LDT. It was changed in 2002 in:

  0bbed3beb4f2 ("[PATCH] Thread-Local Storage (TLS) support")

(commit from the historical tree), like this:

-		/* load_LDT, if either the previous or next thread
-		 * has a non-default LDT.
+		/*
+		 * load the LDT, if the LDT is different:
		 */
-		if (next->context.size+prev->context.size)
+		if (unlikely(prev->context.ldt != next->context.ldt))
			load_LDT(&next->context);

The current code is unlikely to avoid any LDT reloads, since different
mms won't share an LDT.

When we redo lazy mode to stop flush IPIs without switching to
init_mm, though, the current logic would become incorrect: it will
be possible to have real_prev == next but nonetheless have a stale
LDT descriptor.

Simplify the code to update LDTR if either the previous or the next
mm has an LDT, i.e. effectively restore the historical logic..
While we're at it, clean up the code by moving all the ifdeffery to
a header where it belongs.

Signed-off-by: Andy Lutomirski <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/2a859ac01245f9594c58f9d0a8b2ed8a7cd2507e.1498022414.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions