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 <luto@kernel.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/2a859ac01245f9594c58f9d0a8b2ed8a7cd2507e.1498022414.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 2 insertions, 18 deletions

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 2a5e851..b2485d6 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -148,25 +148,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		     real_prev != &init_mm);
 	cpumask_clear_cpu(cpu, mm_cpumask(real_prev));
 
-	/* Load per-mm CR4 state */
+	/* Load per-mm CR4 and LDTR state */
 	load_mm_cr4(next);
-
-#ifdef CONFIG_MODIFY_LDT_SYSCALL
-	/*
-	 * Load the LDT, if the LDT is different.
-	 *
-	 * It's possible that prev->context.ldt doesn't match
-	 * the LDT register.  This can happen if leave_mm(prev)
-	 * was called and then modify_ldt changed
-	 * prev->context.ldt but suppressed an IPI to this CPU.
-	 * In this case, prev->context.ldt != NULL, because we
-	 * never set context.ldt to NULL while the mm still
-	 * exists.  That means that next->context.ldt !=
-	 * prev->context.ldt, because mms never share an LDT.
-	 */
-	if (unlikely(real_prev->context.ldt != next->context.ldt))
-		load_mm_ldt(next);
-#endif
+	switch_ldt(real_prev, next);
 }
 
 /*