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, 4 insertions, 0 deletions

diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index 80fa36be2670..eed218a3fd48 100644
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -82,6 +82,10 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on)
 	/* Update the TSS */
 	tss = this_cpu_ptr(&cpu_tss_rw);
 	memcpy(tss->io_bitmap, t->io_bitmap_ptr, bytes_updated);
+	/* Store the new end of the zero bits */
+	tss->io_bitmap_prev_max = bytes;
+	/* Make the bitmap base in the TSS valid */
+	tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_VALID;
 	/* Make sure the TSS limit covers the I/O bitmap. */
 	refresh_tss_limit();