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Date: Fri, 27 Feb 2009 13:25:21 -0800
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Ingo Molnar <mingo@...e.hu>
CC: the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH 1/3] x86-32: use non-lazy io bitmap context switching
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@...rix.com>
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index c7a98f7..7613950 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 e41980a..99c4d30 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 31ec4e8..01ff913 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -246,11 +246,8 @@ 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();
}
@@ -456,34 +453,19 @@ __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 3d05abf..e447aa8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -119,47 +119,6 @@ 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 = ¤t->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
@@ -310,11 +269,6 @@ 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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