Commit db949bba authored by Jeremy Fitzhardinge's avatar Jeremy Fitzhardinge Committed by Ingo Molnar
Browse files

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: default avatarJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 645af4e9

arch/x86/include/asm/processor.h

+0 −6
Original line number Diff line number Diff line
@@ -248,7 +248,6 @@ struct x86_hw_tss { 
#define IO_BITMAP_LONGS			(IO_BITMAP_BYTES/sizeof(long))

#define IO_BITMAP_OFFSET		offsetof(struct tss_struct, io_bitmap)

#define INVALID_IO_BITMAP_OFFSET	0x8000

#define INVALID_IO_BITMAP_OFFSET_LAZY	0x9000



struct tss_struct {

	/*
@@ -263,11 +262,6 @@ struct tss_struct { 
	 * be within the limit.

	 */

	unsigned long		io_bitmap[IO_BITMAP_LONGS + 1];

	/*

	 * Cache the current maximum and the last task that used the bitmap:

	 */

	unsigned long		io_bitmap_max;

	struct thread_struct	*io_bitmap_owner;



	/*

	 * .. and then another 0x100 bytes for the emergency kernel stack:

arch/x86/kernel/ioport.c

+0 −11
Original line number Diff line number Diff line
@@ -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();

arch/x86/kernel/process_32.c

+9 −27
Original line number Diff line number Diff line
@@ -248,11 +248,8 @@ void exit_thread(void) 
		/*

		 * Careful, clear this in the TSS too:

		 */

		memset(tss->io_bitmap, 0xff, tss->io_bitmap_max);

		memset(tss->io_bitmap, 0xff, t->io_bitmap_max);

		t->io_bitmap_max = 0;

		tss->io_bitmap_owner = NULL;

		tss->io_bitmap_max = 0;

		tss->x86_tss.io_bitmap_base = INVALID_IO_BITMAP_OFFSET;

		put_cpu();

	}
@@ -458,34 +455,19 @@ __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p, 
			hard_enable_TSC();

	}



	if (!test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {

	if (test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {

		/*

		 * Disable the bitmap via an invalid offset. We still cache

		 * the previous bitmap owner and the IO bitmap contents:

		 * Copy the relevant range of the IO bitmap.

		 * Normally this is 128 bytes or less:

		 */

		tss->x86_tss.io_bitmap_base = INVALID_IO_BITMAP_OFFSET;

		return;

	}



	if (likely(next == tss->io_bitmap_owner)) {

		memcpy(tss->io_bitmap, next->io_bitmap_ptr,

		       max(prev->io_bitmap_max, next->io_bitmap_max));

	} else if (test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) {

		/*

		 * Previous owner of the bitmap (hence the bitmap content)

		 * matches the next task, we dont have to do anything but

		 * to set a valid offset in the TSS:

		 * Clear any possible leftover bits:

		 */

		tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET;

		return;

		memset(tss->io_bitmap, 0xff, prev->io_bitmap_max);

	}

	/*

	 * Lazy TSS's I/O bitmap copy. We set an invalid offset here

	 * and we let the task to get a GPF in case an I/O instruction

	 * is performed.  The handler of the GPF will verify that the

	 * faulting task has a valid I/O bitmap and, it true, does the

	 * real copy and restart the instruction.  This will save us

	 * redundant copies when the currently switched task does not

	 * perform any I/O during its timeslice.

	 */

	tss->x86_tss.io_bitmap_base = INVALID_IO_BITMAP_OFFSET_LAZY;

}



/*

arch/x86/kernel/traps.c

+0 −46
Original line number Diff line number Diff line
@@ -118,47 +118,6 @@ die_if_kernel(const char *str, struct pt_regs *regs, long err) 
	if (!user_mode_vm(regs))

		die(str, regs, err);

}



/*

 * Perform the lazy TSS's I/O bitmap copy. If the TSS has an

 * invalid offset set (the LAZY one) and the faulting thread has

 * a valid I/O bitmap pointer, we copy the I/O bitmap in the TSS,

 * we set the offset field correctly and return 1.

 */

static int lazy_iobitmap_copy(void)

{

	struct thread_struct *thread;

	struct tss_struct *tss;

	int cpu;



	cpu = get_cpu();

	tss = &per_cpu(init_tss, cpu);

	thread = &current->thread;



	if (tss->x86_tss.io_bitmap_base == INVALID_IO_BITMAP_OFFSET_LAZY &&

	    thread->io_bitmap_ptr) {

		memcpy(tss->io_bitmap, thread->io_bitmap_ptr,

		       thread->io_bitmap_max);

		/*

		 * If the previously set map was extending to higher ports

		 * than the current one, pad extra space with 0xff (no access).

		 */

		if (thread->io_bitmap_max < tss->io_bitmap_max) {

			memset((char *) tss->io_bitmap +

				thread->io_bitmap_max, 0xff,

				tss->io_bitmap_max - thread->io_bitmap_max);

		}

		tss->io_bitmap_max = thread->io_bitmap_max;

		tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET;

		tss->io_bitmap_owner = thread;

		put_cpu();



		return 1;

	}

	put_cpu();



	return 0;

}

#endif



static void __kprobes
@@ -309,11 +268,6 @@ do_general_protection(struct pt_regs *regs, long error_code) 
	conditional_sti(regs);



#ifdef CONFIG_X86_32

	if (lazy_iobitmap_copy()) {

		/* restart the faulting instruction */

		return;

	}



	if (regs->flags & X86_VM_MASK)

		goto gp_in_vm86;

#endif

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