x86/io: Speedup schedule out of I/O bitmap user

There is no requirement to update the TSS I/O bitmap when a thread using it is
scheduled out and the incoming thread does not use it.

For the permission check based on the TSS I/O bitmap the CPU calculates the memory
location of the I/O bitmap by the address of the TSS and the io_bitmap_base member
of the tss_struct. The easiest way to invalidate the I/O bitmap is to switch the
offset to an address outside of the TSS limit.

If an I/O instruction is issued from user space the TSS limit causes #GP to be
raised in the same was as valid I/O bitmap with all bits set to 1 would do.

This removes the extra work when an I/O bitmap using task is scheduled out
and puts the burden on the rare I/O bitmap users when they are scheduled
in.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index d52ec1a82120..8c1000a57b55 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1860,7 +1860,8 @@ void cpu_init(void)
 
 	/* Initialize the TSS. */
 	tss_setup_ist(tss);
-	tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET;
+	tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_INVALID;
+	tss->io_bitmap_prev_max = 0;
 	memset(tss->io_bitmap, 0xff, sizeof(tss->io_bitmap));
 	set_tss_desc(cpu, &get_cpu_entry_area(cpu)->tss.x86_tss);