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>

4 files changed, 9 insertions, 90 deletions

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index c7a98f73821..76139506c3e 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -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:
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();
 
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 646da41a620..a59314e877f 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -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;
 }
 
 /*
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c05430ac1b4..a1d288327ff 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -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