x86-32: use non-lazy io bitmap context switching

Impact: remove 32-bit optimization to prepare unification

x86-32 and -64 differ in the way they context-switch tasks
with io permission bitmaps.  x86-64 simply copies the next
tasks io bitmap into place (if any) on context switch.  x86-32
invalidates the bitmap on context switch, so that the next
IO instruction will fault; at that point it installs the
appropriate IO bitmap.

This makes context switching IO-bitmap-using tasks a bit more
less expensive, at the cost of making the next IO instruction
slower due to the extra fault.  This tradeoff only makes sense
if IO-bitmap-using processes are relatively common, but they
don't actually use IO instructions very often.

However, in a typical desktop system, the only process likely
to be using IO bitmaps is the X server, and nothing at all on
a server.  Therefore the lazy context switch doesn't really win
all that much, and its just a gratuitious difference from
64-bit code.

This patch removes the lazy context switch, with a view to
unifying this code in a later change.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 0 insertions, 11 deletions

diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index e41980a373a..99c4d308f16 100644
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -85,19 +85,8 @@ asmlinkage long sys_ioperm(unsigned long from, unsigned long num, int turn_on)
 
 	t->io_bitmap_max = bytes;
 
-#ifdef CONFIG_X86_32
-	/*
-	 * Sets the lazy trigger so that the next I/O operation will
-	 * reload the correct bitmap.
-	 * Reset the owner so that a process switch will not set
-	 * tss->io_bitmap_base to IO_BITMAP_OFFSET.
-	 */
-	tss->x86_tss.io_bitmap_base = INVALID_IO_BITMAP_OFFSET_LAZY;
-	tss->io_bitmap_owner = NULL;
-#else
 	/* Update the TSS: */
 	memcpy(tss->io_bitmap, t->io_bitmap_ptr, bytes_updated);
-#endif
 
 	put_cpu();