Re: [PATCH 0/5] i387: stable kernel backport

raphael
2012-02-23T02:25:34Z

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On 23.02.2012 08:37, Greg Kroah-Hartman wrote:
> On Wed, Feb 22, 2012 at 04:25:42PM -0800, Linus Torvalds wrote:
>> On Wed, Feb 22, 2012 at 4:14 PM, H. Peter Anvin <hpa@zytor.com>
>> wrote:
>> >
>> > Immediate failure, still.
>>
>> Damn, I'll have to look closer at what else is missing then.
>>
>> I really didn't want to back-port the whole series as-is, but that
>> would certainly have been easier.

Hello,

Thank you for backporting this patchset to -stable. FWIW, the test
machine I had been working with has an uptime of 4 days now, with the
patchset in attachment applied on top of 3.2.6, so if it were
unpractical to trim it down further you can find solace in that it does
not break anything.

I'll be glad to help test any new patchset as well!

Also, the issue started before 2.6.30, 2.6.39 had the same issue, and
probably 2.6.38 even I did not test it for a while. 2.6.37 was the last
known "safe" release WRT this bug.

>
> I can do that, it wouldn't be that hard, just give me the git commit
> ids to apply.
>
> I can do a release with just these fixes in it if you think that
> would
> make it easier to test/verify.
>
> thanks,
>
> greg k-h

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From be98c2cdb15ba26148cd2bd58a857d4f7759ed38 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 13 Feb 2012 13:47:25 -0800
Subject: [PATCH] i387: math_state_restore() isn't called from asm

It was marked asmlinkage for some really old and stale legacy reasons.
Fix that and the equally stale comment.

Noticed when debugging the irq_fpu_usable() bugs.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 2 +-
arch/x86/kernel/traps.c | 6 +++---
2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 6919e93..a5c7ae5 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -29,7 +29,7 @@ extern unsigned int sig_xstate_size;
extern void fpu_init(void);
extern void mxcsr_feature_mask_init(void);
extern int init_fpu(struct task_struct *child);
-extern asmlinkage void math_state_restore(void);
+extern void math_state_restore(void);
extern void __math_state_restore(void);
extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 482ec3a..982433b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -599,10 +599,10 @@ void __math_state_restore(void)
* Careful.. There are problems with IBM-designed IRQ13 behaviour.
* Don't touch unless you *really* know how it works.
*
- * Must be called with kernel preemption disabled (in this case,
- * local interrupts are disabled at the call-site in entry.S).
+ * Must be called with kernel preemption disabled (eg with local
+ * local interrupts as in the case of do_device_not_available).
*/
-asmlinkage void math_state_restore(void)
+void math_state_restore(void)
{
struct thread_info *thread = current_thread_info();
struct task_struct *tsk = thread->task;
--
1.7.6.5

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From 5b1cbac37798805c1fee18c8cebe5c0a13975b17 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 13 Feb 2012 13:56:14 -0800
Subject: [PATCH] i387: make irq_fpu_usable() tests more robust

Some code - especially the crypto layer - wants to use the x86
FP/MMX/AVX register set in what may be interrupt (typically softirq)
context.

That *can* be ok, but the tests for when it was ok were somewhat
suspect. We cannot touch the thread-specific status bits either, so
we'd better check that we're not going to try to save FP state or
anything like that.

Now, it may be that the TS bit is always cleared *before* we set the
USEDFPU bit (and only set when we had already cleared the USEDFP
before), so the TS bit test may actually have been sufficient, but it
certainly was not obviously so.

So this explicitly verifies that we will not touch the TS_USEDFPU bit,
and adds a few related sanity-checks. Because it seems that somehow
AES-NI is corrupting user FP state. The cause is not clear, and this
patch doesn't fix it, but while debugging it I really wanted the code to
be more obviously correct and robust.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 54 ++++++++++++++++++++++++++++++++++++------
arch/x86/kernel/traps.c | 1 +
2 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index a5c7ae5..a295718 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -307,9 +307,54 @@ static inline void __clear_fpu(struct task_struct *tsk)
}
}

+/*
+ * Were we in an interrupt that interrupted kernel mode?
+ *
+ * We can do a kernel_fpu_begin/end() pair *ONLY* if that
+ * pair does nothing at all: TS_USEDFPU must be clear (so
+ * that we don't try to save the FPU state), and TS must
+ * be set (so that the clts/stts pair does nothing that is
+ * visible in the interrupted kernel thread).
+ */
+static inline bool interrupted_kernel_fpu_idle(void)
+{
+ return !(current_thread_info()->status & TS_USEDFPU) &&
+ (read_cr0() & X86_CR0_TS);
+}
+
+/*
+ * Were we in user mode (or vm86 mode) when we were
+ * interrupted?
+ *
+ * Doing kernel_fpu_begin/end() is ok if we are running
+ * in an interrupt context from user mode - we'll just
+ * save the FPU state as required.
+ */
+static inline bool interrupted_user_mode(void)
+{
+ struct pt_regs *regs = get_irq_regs();
+ return regs && user_mode_vm(regs);
+}
+
+/*
+ * Can we use the FPU in kernel mode with the
+ * whole "kernel_fpu_begin/end()" sequence?
+ *
+ * It's always ok in process context (ie "not interrupt")
+ * but it is sometimes ok even from an irq.
+ */
+static inline bool irq_fpu_usable(void)
+{
+ return !in_interrupt() ||
+ interrupted_user_mode() ||
+ interrupted_kernel_fpu_idle();
+}
+
static inline void kernel_fpu_begin(void)
{
struct thread_info *me = current_thread_info();
+
+ WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
if (me->status & TS_USEDFPU)
__save_init_fpu(me->task);
@@ -323,14 +368,6 @@ static inline void kernel_fpu_end(void)
preempt_enable();
}

-static inline bool irq_fpu_usable(void)
-{
- struct pt_regs *regs;
-
- return !in_interrupt() || !(regs = get_irq_regs()) || \
- user_mode(regs) || (read_cr0() & X86_CR0_TS);
-}
-
/*
* Some instructions like VIA's padlock instructions generate a spurious
* DNA fault but don't modify SSE registers. And these instructions
@@ -367,6 +404,7 @@ static inline void irq_ts_restore(int TS_state)
*/
static inline void save_init_fpu(struct task_struct *tsk)
{
+ WARN_ON_ONCE(task_thread_info(tsk)->status & TS_USEDFPU);
preempt_disable();
__save_init_fpu(tsk);
stts();
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 982433b..8ba27db 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -631,6 +631,7 @@ EXPORT_SYMBOL_GPL(math_state_restore);
dotraplinkage void __kprobes
do_device_not_available(struct pt_regs *regs, long error_code)
{
+ WARN_ON_ONCE(!user_mode_vm(regs));
#ifdef CONFIG_MATH_EMULATION
if (read_cr0() & X86_CR0_EM) {
struct math_emu_info info = { };
--
1.7.6.5

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From c38e23456278e967f094b08247ffc3711b1029b2 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 15 Feb 2012 08:05:18 -0800
Subject: [PATCH] i387: fix sense of sanity check

The check for save_init_fpu() (introduced in commit 5b1cbac37798: "i387:
make irq_fpu_usable() tests more robust") was the wrong way around, but
I hadn't noticed, because my "tests" were bogus: the FPU exceptions are
disabled by default, so even doing a divide by zero never actually
triggers this code at all unless you do extra work to enable them.

So if anybody did enable them, they'd get one spurious warning.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index a295718..727c1dd 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -404,7 +404,7 @@ static inline void irq_ts_restore(int TS_state)
*/
static inline void save_init_fpu(struct task_struct *tsk)
{
- WARN_ON_ONCE(task_thread_info(tsk)->status & TS_USEDFPU);
+ WARN_ON_ONCE(!(task_thread_info(tsk)->status & TS_USEDFPU));
preempt_disable();
__save_init_fpu(tsk);
stts();
--
1.7.6.5

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From 15d8791cae75dca27bfda8ecfe87dca9379d6bb0 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Feb 2012 09:15:04 -0800
Subject: [PATCH] i387: fix x86-64 preemption-unsafe user stack save/restore

Commit 5b1cbac37798 ("i387: make irq_fpu_usable() tests more robust")
added a sanity check to the #NM handler to verify that we never cause
the "Device Not Available" exception in kernel mode.

However, that check actually pinpointed a (fundamental) race where we do
cause that exception as part of the signal stack FPU state save/restore
code.

Because we use the floating point instructions themselves to save and
restore state directly from user mode, we cannot do that atomically with
testing the TS_USEDFPU bit: the user mode access itself may cause a page
fault, which causes a task switch, which saves and restores the FP/MMX
state from the kernel buffers.

This kind of "recursive" FP state save is fine per se, but it means that
when the signal stack save/restore gets restarted, it will now take the
'#NM' exception we originally tried to avoid. With preemption this can
happen even without the page fault - but because of the user access, we
cannot just disable preemption around the save/restore instruction.

There are various ways to solve this, including using the
"enable/disable_page_fault()" helpers to not allow page faults at all
during the sequence, and fall back to copying things by hand without the
use of the native FP state save/restore instructions.

However, the simplest thing to do is to just allow the #NM from kernel
space, but fix the race in setting and clearing CR0.TS that this all
exposed: the TS bit changes and the TS_USEDFPU bit absolutely have to be
atomic wrt scheduling, so while the actual state save/restore can be
interrupted and restarted, the act of actually clearing/setting CR0.TS
and the TS_USEDFPU bit together must not.

Instead of just adding random "preempt_disable/enable()" calls to what
is already excessively ugly code, this introduces some helper functions
that mostly mirror the "kernel_fpu_begin/end()" functionality, just for
the user state instead.

Those helper functions should probably eventually replace the other
ad-hoc CR0.TS and TS_USEDFPU tests too, but I'll need to think about it
some more: the task switching functionality in particular needs to
expose the difference between the 'prev' and 'next' threads, while the
new helper functions intentionally were written to only work with
'current'.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 42 ++++++++++++++++++++++++++++++++++++++++++
arch/x86/kernel/traps.c | 1 -
arch/x86/kernel/xsave.c | 10 +++-------
3 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 727c1dd..f704be2 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -400,6 +400,48 @@ static inline void irq_ts_restore(int TS_state)
}

/*
+ * The question "does this thread have fpu access?"
+ * is slightly racy, since preemption could come in
+ * and revoke it immediately after the test.
+ *
+ * However, even in that very unlikely scenario,
+ * we can just assume we have FPU access - typically
+ * to save the FP state - we'll just take a #NM
+ * fault and get the FPU access back.
+ *
+ * The actual user_fpu_begin/end() functions
+ * need to be preemption-safe, though.
+ *
+ * NOTE! user_fpu_end() must be used only after you
+ * have saved the FP state, and user_fpu_begin() must
+ * be used only immediately before restoring it.
+ * These functions do not do any save/restore on
+ * their own.
+ */
+static inline int user_has_fpu(void)
+{
+ return current_thread_info()->status & TS_USEDFPU;
+}
+
+static inline void user_fpu_end(void)
+{
+ preempt_disable();
+ current_thread_info()->status &= ~TS_USEDFPU;
+ stts();
+ preempt_enable();
+}
+
+static inline void user_fpu_begin(void)
+{
+ preempt_disable();
+ if (!user_has_fpu()) {
+ clts();
+ current_thread_info()->status |= TS_USEDFPU;
+ }
+ preempt_enable();
+}
+
+/*
* These disable preemption on their own and are safe
*/
static inline void save_init_fpu(struct task_struct *tsk)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 8ba27db..982433b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -631,7 +631,6 @@ EXPORT_SYMBOL_GPL(math_state_restore);
dotraplinkage void __kprobes
do_device_not_available(struct pt_regs *regs, long error_code)
{
- WARN_ON_ONCE(!user_mode_vm(regs));
#ifdef CONFIG_MATH_EMULATION
if (read_cr0() & X86_CR0_EM) {
struct math_emu_info info = { };
diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index a391134..86f1f09 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -168,7 +168,7 @@ int save_i387_xstate(void __user *buf)
if (!used_math())
return 0;

- if (task_thread_info(tsk)->status & TS_USEDFPU) {
+ if (user_has_fpu()) {
if (use_xsave())
err = xsave_user(buf);
else
@@ -176,8 +176,7 @@ int save_i387_xstate(void __user *buf)

if (err)
return err;
- task_thread_info(tsk)->status &= ~TS_USEDFPU;
- stts();
+ user_fpu_end();
} else {
sanitize_i387_state(tsk);
if (__copy_to_user(buf, &tsk->thread.fpu.state->fxsave,
@@ -292,10 +291,7 @@ int restore_i387_xstate(void __user *buf)
return err;
}

- if (!(task_thread_info(current)->status & TS_USEDFPU)) {
- clts();
- task_thread_info(current)->status |= TS_USEDFPU;
- }
+ user_fpu_begin();
if (use_xsave())
err = restore_user_xstate(buf);
else
--
1.7.6.5

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From b6c66418dcad0fcf83cd1d0a39482db37bf4fc41 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Feb 2012 12:22:48 -0800
Subject: [PATCH] i387: move TS_USEDFPU clearing out of __save_init_fpu and
into callers

Touching TS_USEDFPU without touching CR0.TS is confusing, so don't do
it. By moving it into the callers, we always do the TS_USEDFPU next to
the CR0.TS accesses in the source code, and it's much easier to see how
the two go hand in hand.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 9 ++++++---
1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index f704be2..1e12c2d 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -259,7 +259,6 @@ static inline void fpu_save_init(struct fpu *fpu)
static inline void __save_init_fpu(struct task_struct *tsk)
{
fpu_save_init(&tsk->thread.fpu);
- task_thread_info(tsk)->status &= ~TS_USEDFPU;
}

static inline int fpu_fxrstor_checking(struct fpu *fpu)
@@ -290,6 +289,7 @@ static inline void __unlazy_fpu(struct task_struct *tsk)
{
if (task_thread_info(tsk)->status & TS_USEDFPU) {
__save_init_fpu(tsk);
+ task_thread_info(tsk)->status &= ~TS_USEDFPU;
stts();
} else
tsk->fpu_counter = 0;
@@ -356,9 +356,11 @@ static inline void kernel_fpu_begin(void)

WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
- if (me->status & TS_USEDFPU)
+ if (me->status & TS_USEDFPU) {
__save_init_fpu(me->task);
- else
+ me->status &= ~TS_USEDFPU;
+ /* We do 'stts()' in kernel_fpu_end() */
+ } else
clts();
}

@@ -449,6 +451,7 @@ static inline void save_init_fpu(struct task_struct *tsk)
WARN_ON_ONCE(!(task_thread_info(tsk)->status & TS_USEDFPU));
preempt_disable();
__save_init_fpu(tsk);
+ task_thread_info(tsk)->status &= ~TS_USEDFPU;
stts();
preempt_enable();
}
--
1.7.6.5

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From 6d59d7a9f5b723a7ac1925c136e93ec83c0c3043 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Feb 2012 13:33:12 -0800
Subject: [PATCH] i387: don't ever touch TS_USEDFPU directly, use helper
functions

This creates three helper functions that do the TS_USEDFPU accesses, and
makes everybody that used to do it by hand use those helpers instead.

In addition, there's a couple of helper functions for the "change both
CR0.TS and TS_USEDFPU at the same time" case, and the places that do
that together have been changed to use those. That means that we have
fewer random places that open-code this situation.

The intent is partly to clarify the code without actually changing any
semantics yet (since we clearly still have some hard to reproduce bug in
this area), but also to make it much easier to use another approach
entirely to caching the CR0.TS bit for software accesses.

Right now we use a bit in the thread-info 'status' variable (this patch
does not change that), but we might want to make it a full field of its
own or even make it a per-cpu variable.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 75 +++++++++++++++++++++++++++++++-----------
arch/x86/kernel/traps.c | 2 +-
arch/x86/kernel/xsave.c | 2 +-
arch/x86/kvm/vmx.c | 2 +-
4 files changed, 58 insertions(+), 23 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 1e12c2d..548b2c0 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -280,6 +280,47 @@ static inline int restore_fpu_checking(struct task_struct *tsk)
}

/*
+ * Software FPU state helpers. Careful: these need to
+ * be preemption protection *and* they need to be
+ * properly paired with the CR0.TS changes!
+ */
+static inline int __thread_has_fpu(struct thread_info *ti)
+{
+ return ti->status & TS_USEDFPU;
+}
+
+/* Must be paired with an 'stts' after! */
+static inline void __thread_clear_has_fpu(struct thread_info *ti)
+{
+ ti->status &= ~TS_USEDFPU;
+}
+
+/* Must be paired with a 'clts' before! */
+static inline void __thread_set_has_fpu(struct thread_info *ti)
+{
+ ti->status |= TS_USEDFPU;
+}
+
+/*
+ * Encapsulate the CR0.TS handling together with the
+ * software flag.
+ *
+ * These generally need preemption protection to work,
+ * do try to avoid using these on their own.
+ */
+static inline void __thread_fpu_end(struct thread_info *ti)
+{
+ __thread_clear_has_fpu(ti);
+ stts();
+}
+
+static inline void __thread_fpu_begin(struct thread_info *ti)
+{
+ clts();
+ __thread_set_has_fpu(ti);
+}
+
+/*
* Signal frame handlers...
*/
extern int save_i387_xstate(void __user *buf);
@@ -287,23 +328,21 @@ extern int restore_i387_xstate(void __user *buf);

static inline void __unlazy_fpu(struct task_struct *tsk)
{
- if (task_thread_info(tsk)->status & TS_USEDFPU) {
+ if (__thread_has_fpu(task_thread_info(tsk))) {
__save_init_fpu(tsk);
- task_thread_info(tsk)->status &= ~TS_USEDFPU;
- stts();
+ __thread_fpu_end(task_thread_info(tsk));
} else
tsk->fpu_counter = 0;
}

static inline void __clear_fpu(struct task_struct *tsk)
{
- if (task_thread_info(tsk)->status & TS_USEDFPU) {
+ if (__thread_has_fpu(task_thread_info(tsk))) {
/* Ignore delayed exceptions from user space */
asm volatile("1: fwait\n"
"2:\n"
_ASM_EXTABLE(1b, 2b));
- task_thread_info(tsk)->status &= ~TS_USEDFPU;
- stts();
+ __thread_fpu_end(task_thread_info(tsk));
}
}

@@ -311,14 +350,14 @@ static inline void __clear_fpu(struct task_struct *tsk)
* Were we in an interrupt that interrupted kernel mode?
*
* We can do a kernel_fpu_begin/end() pair *ONLY* if that
- * pair does nothing at all: TS_USEDFPU must be clear (so
+ * pair does nothing at all: the thread must not have fpu (so
* that we don't try to save the FPU state), and TS must
* be set (so that the clts/stts pair does nothing that is
* visible in the interrupted kernel thread).
*/
static inline bool interrupted_kernel_fpu_idle(void)
{
- return !(current_thread_info()->status & TS_USEDFPU) &&
+ return !__thread_has_fpu(current_thread_info()) &&
(read_cr0() & X86_CR0_TS);
}

@@ -356,9 +395,9 @@ static inline void kernel_fpu_begin(void)

WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
- if (me->status & TS_USEDFPU) {
+ if (__thread_has_fpu(me)) {
__save_init_fpu(me->task);
- me->status &= ~TS_USEDFPU;
+ __thread_clear_has_fpu(me);
/* We do 'stts()' in kernel_fpu_end() */
} else
clts();
@@ -422,24 +461,21 @@ static inline void irq_ts_restore(int TS_state)
*/
static inline int user_has_fpu(void)
{
- return current_thread_info()->status & TS_USEDFPU;
+ return __thread_has_fpu(current_thread_info());
}

static inline void user_fpu_end(void)
{
preempt_disable();
- current_thread_info()->status &= ~TS_USEDFPU;
- stts();
+ __thread_fpu_end(current_thread_info());
preempt_enable();
}

static inline void user_fpu_begin(void)
{
preempt_disable();
- if (!user_has_fpu()) {
- clts();
- current_thread_info()->status |= TS_USEDFPU;
- }
+ if (!user_has_fpu())
+ __thread_fpu_begin(current_thread_info());
preempt_enable();
}

@@ -448,11 +484,10 @@ static inline void user_fpu_begin(void)
*/
static inline void save_init_fpu(struct task_struct *tsk)
{
- WARN_ON_ONCE(!(task_thread_info(tsk)->status & TS_USEDFPU));
+ WARN_ON_ONCE(!__thread_has_fpu(task_thread_info(tsk)));
preempt_disable();
__save_init_fpu(tsk);
- task_thread_info(tsk)->status &= ~TS_USEDFPU;
- stts();
+ __thread_fpu_end(task_thread_info(tsk));
preempt_enable();
}

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 982433b..fc676e4 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -588,7 +588,7 @@ void __math_state_restore(void)
return;
}

- thread->status |= TS_USEDFPU; /* So we fnsave on switch_to() */
+ __thread_set_has_fpu(thread); /* clts in caller! */
tsk->fpu_counter++;
}

diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 86f1f09..a0bcd0d 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -47,7 +47,7 @@ void __sanitize_i387_state(struct task_struct *tsk)
if (!fx)
return;

- BUG_ON(task_thread_info(tsk)->status & TS_USEDFPU);
+ BUG_ON(__thread_has_fpu(task_thread_info(tsk)));

xstate_bv = tsk->thread.fpu.state->xsave.xsave_hdr.xstate_bv;

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index d29216c..36091dd 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1457,7 +1457,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
#ifdef CONFIG_X86_64
wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
#endif
- if (current_thread_info()->status & TS_USEDFPU)
+ if (__thread_has_fpu(current_thread_info()))
clts();
load_gdt(&__get_cpu_var(host_gdt));
}
--
1.7.6.5

[-- Attachment #8: fpu-patch-9.diff --]
[-- Type: text/plain, Size: 7588 bytes --]

arch/x86/include/asm/i387.h | 44 ++++++++++++++++++------------------
arch/x86/include/asm/processor.h | 1 +
arch/x86/include/asm/thread_info.h | 2 -
arch/x86/kernel/traps.c | 11 ++++-----
arch/x86/kernel/xsave.c | 2 +-
arch/x86/kvm/vmx.c | 2 +-
6 files changed, 30 insertions(+), 32 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 01b115d86770..f5376676f89c 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -264,21 +264,21 @@ static inline int restore_fpu_checking(struct task_struct *tsk)
* be preemption protection *and* they need to be
* properly paired with the CR0.TS changes!
*/
-static inline int __thread_has_fpu(struct thread_info *ti)
+static inline int __thread_has_fpu(struct task_struct *tsk)
{
- return ti->status & TS_USEDFPU;
+ return tsk->thread.has_fpu;
}

/* Must be paired with an 'stts' after! */
-static inline void __thread_clear_has_fpu(struct thread_info *ti)
+static inline void __thread_clear_has_fpu(struct task_struct *tsk)
{
- ti->status &= ~TS_USEDFPU;
+ tsk->thread.has_fpu = 0;
}

/* Must be paired with a 'clts' before! */
-static inline void __thread_set_has_fpu(struct thread_info *ti)
+static inline void __thread_set_has_fpu(struct task_struct *tsk)
{
- ti->status |= TS_USEDFPU;
+ tsk->thread.has_fpu = 1;
}

/*
@@ -288,16 +288,16 @@ static inline void __thread_set_has_fpu(struct thread_info *ti)
* These generally need preemption protection to work,
* do try to avoid using these on their own.
*/
-static inline void __thread_fpu_end(struct thread_info *ti)
+static inline void __thread_fpu_end(struct task_struct *tsk)
{
- __thread_clear_has_fpu(ti);
+ __thread_clear_has_fpu(tsk);
stts();
}

-static inline void __thread_fpu_begin(struct thread_info *ti)
+static inline void __thread_fpu_begin(struct task_struct *tsk)
{
clts();
- __thread_set_has_fpu(ti);
+ __thread_set_has_fpu(tsk);
}

/*
@@ -308,21 +308,21 @@ extern int restore_i387_xstate(void __user *buf);

static inline void __unlazy_fpu(struct task_struct *tsk)
{
- if (__thread_has_fpu(task_thread_info(tsk))) {
+ if (__thread_has_fpu(tsk)) {
__save_init_fpu(tsk);
- __thread_fpu_end(task_thread_info(tsk));
+ __thread_fpu_end(tsk);
} else
tsk->fpu_counter = 0;
}

static inline void __clear_fpu(struct task_struct *tsk)
{
- if (__thread_has_fpu(task_thread_info(tsk))) {
+ if (__thread_has_fpu(tsk)) {
/* Ignore delayed exceptions from user space */
asm volatile("1: fwait\n"
"2:\n"
_ASM_EXTABLE(1b, 2b));
- __thread_fpu_end(task_thread_info(tsk));
+ __thread_fpu_end(tsk);
}
}

@@ -337,7 +337,7 @@ static inline void __clear_fpu(struct task_struct *tsk)
*/
static inline bool interrupted_kernel_fpu_idle(void)
{
- return !__thread_has_fpu(current_thread_info()) &&
+ return !__thread_has_fpu(current) &&
(read_cr0() & X86_CR0_TS);
}

@@ -371,12 +371,12 @@ static inline bool irq_fpu_usable(void)

static inline void kernel_fpu_begin(void)
{
- struct thread_info *me = current_thread_info();
+ struct task_struct *me = current;

WARN_ON_ONCE(!irq_fpu_usable());
preempt_disable();
if (__thread_has_fpu(me)) {
- __save_init_fpu(me->task);
+ __save_init_fpu(me);
__thread_clear_has_fpu(me);
/* We do 'stts()' in kernel_fpu_end() */
} else
@@ -441,13 +441,13 @@ static inline void irq_ts_restore(int TS_state)
*/
static inline int user_has_fpu(void)
{
- return __thread_has_fpu(current_thread_info());
+ return __thread_has_fpu(current);
}

static inline void user_fpu_end(void)
{
preempt_disable();
- __thread_fpu_end(current_thread_info());
+ __thread_fpu_end(current);
preempt_enable();
}

@@ -455,7 +455,7 @@ static inline void user_fpu_begin(void)
{
preempt_disable();
if (!user_has_fpu())
- __thread_fpu_begin(current_thread_info());
+ __thread_fpu_begin(current);
preempt_enable();
}

@@ -464,10 +464,10 @@ static inline void user_fpu_begin(void)
*/
static inline void save_init_fpu(struct task_struct *tsk)
{
- WARN_ON_ONCE(!__thread_has_fpu(task_thread_info(tsk)));
+ WARN_ON_ONCE(!__thread_has_fpu(tsk));
preempt_disable();
__save_init_fpu(tsk);
- __thread_fpu_end(task_thread_info(tsk));
+ __thread_fpu_end(tsk);
preempt_enable();
}

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index aa9088c26931..f7c89e231c6c 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -454,6 +454,7 @@ struct thread_struct {
unsigned long trap_no;
unsigned long error_code;
/* floating point and extended processor state */
+ unsigned long has_fpu;
struct fpu fpu;
#ifdef CONFIG_X86_32
/* Virtual 86 mode info */
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index bc817cd8b443..cfd8144d5527 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -247,8 +247,6 @@ static inline struct thread_info *current_thread_info(void)
* ever touches our thread-synchronous status, so we don't
* have to worry about atomic accesses.
*/
-#define TS_USEDFPU 0x0001 /* FPU was used by this task
- this quantum (SMP) */
#define TS_COMPAT 0x0002 /* 32bit syscall active (64BIT)*/
#define TS_POLLING 0x0004 /* idle task polling need_resched,
skip sending interrupt */
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 4d42300dcd2c..ad25e51f40c4 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -582,12 +582,11 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
*/
void math_state_restore(void)
{
- struct thread_info *thread = current_thread_info();
- struct task_struct *tsk = thread->task;
+ struct task_struct *tsk = current;

/* We need a safe address that is cheap to find and that is already
- in L1. We just brought in "thread->task", so use that */
-#define safe_address (thread->task)
+ in L1. We're just bringing in "tsk->thread.has_fpu", so use that */
+#define safe_address (tsk->thread.has_fpu)

if (!tsk_used_math(tsk)) {
local_irq_enable();
@@ -604,7 +603,7 @@ void math_state_restore(void)
local_irq_disable();
}

- __thread_fpu_begin(thread);
+ __thread_fpu_begin(tsk);

/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
is pending. Clear the x87 state here by setting it to fixed
@@ -620,7 +619,7 @@ void math_state_restore(void)
* Paranoid restore. send a SIGSEGV if we fail to restore the state.
*/
if (unlikely(restore_fpu_checking(tsk))) {
- __thread_fpu_end(thread);
+ __thread_fpu_end(tsk);
force_sig(SIGSEGV, tsk);
return;
}
diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index a0bcd0dbc951..711091114119 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -47,7 +47,7 @@ void __sanitize_i387_state(struct task_struct *tsk)
if (!fx)
return;

- BUG_ON(__thread_has_fpu(task_thread_info(tsk)));
+ BUG_ON(__thread_has_fpu(tsk));

xstate_bv = tsk->thread.fpu.state->xsave.xsave_hdr.xstate_bv;

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 36091dd04b4b..3b4c8d8ad906 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1457,7 +1457,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
#ifdef CONFIG_X86_64
wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
#endif
- if (__thread_has_fpu(current_thread_info()))
+ if (__thread_has_fpu(current))
clts();
load_gdt(&__get_cpu_var(host_gdt));
}

[-- Attachment #9: fpu-patch-81.diff --]
[-- Type: text/plain, Size: 7005 bytes --]

From b3b0870ef3ffed72b92415423da864f440f57ad6 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Feb 2012 15:45:23 -0800
Subject: [PATCH] i387: do not preload FPU state at task switch time

Yes, taking the trap to re-load the FPU/MMX state is expensive, but so
is spending several days looking for a bug in the state save/restore
code. And the preload code has some rather subtle interactions with
both paravirtualization support and segment state restore, so it's not
nearly as simple as it should be.

Also, now that we no longer necessarily depend on a single bit (ie
TS_USEDFPU) for keeping track of the state of the FPU, we migth be able
to do better. If we are really switching between two processes that
keep touching the FP state, save/restore is inevitable, but in the case
of having one process that does most of the FPU usage, we may actually
be able to do much better than the preloading.

In particular, we may be able to keep track of which CPU the process ran
on last, and also per CPU keep track of which process' FP state that CPU
has. For modern CPU's that don't destroy the FPU contents on save time,
that would allow us to do a lazy restore by just re-enabling the
existing FPU state - with no restore cost at all!

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 1 -
arch/x86/kernel/process_32.c | 20 --------------------
arch/x86/kernel/process_64.c | 23 -----------------------
arch/x86/kernel/traps.c | 35 +++++++++++------------------------
4 files changed, 11 insertions(+), 68 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 548b2c0..86974c7 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -30,7 +30,6 @@ extern void fpu_init(void);
extern void mxcsr_feature_mask_init(void);
extern int init_fpu(struct task_struct *child);
extern void math_state_restore(void);
-extern void __math_state_restore(void);
extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);

extern user_regset_active_fn fpregs_active, xfpregs_active;
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 485204f..324cd72 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -299,23 +299,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
*next = &next_p->thread;
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
- bool preload_fpu;

/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */

- /*
- * If the task has used fpu the last 5 timeslices, just do a full
- * restore of the math state immediately to avoid the trap; the
- * chances of needing FPU soon are obviously high now
- */
- preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter > 5;
-
__unlazy_fpu(prev_p);

- /* we're going to use this soon, after a few expensive things */
- if (preload_fpu)
- prefetch(next->fpu.state);
-
/*
* Reload esp0.
*/
@@ -354,11 +342,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
__switch_to_xtra(prev_p, next_p, tss);

- /* If we're going to preload the fpu context, make sure clts
- is run while we're batching the cpu state updates. */
- if (preload_fpu)
- clts();
-
/*
* Leave lazy mode, flushing any hypercalls made here.
* This must be done before restoring TLS segments so
@@ -368,9 +351,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
*/
arch_end_context_switch(next_p);

- if (preload_fpu)
- __math_state_restore();
-
/*
* Restore %gs if needed (which is common)
*/
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 9b9fe4a..992b4e5 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -386,18 +386,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
unsigned fsindex, gsindex;
- bool preload_fpu;
-
- /*
- * If the task has used fpu the last 5 timeslices, just do a full
- * restore of the math state immediately to avoid the trap; the
- * chances of needing FPU soon are obviously high now
- */
- preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter > 5;
-
- /* we're going to use this soon, after a few expensive things */
- if (preload_fpu)
- prefetch(next->fpu.state);

/*
* Reload esp0, LDT and the page table pointer:
@@ -430,10 +418,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
/* Must be after DS reload */
__unlazy_fpu(prev_p);

- /* Make sure cpu is ready for new context */
- if (preload_fpu)
- clts();
-
/*
* Leave lazy mode, flushing any hypercalls made here.
* This must be done before restoring TLS segments so
@@ -492,13 +476,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV))
__switch_to_xtra(prev_p, next_p, tss);

- /*
- * Preload the FPU context, now that we've determined that the
- * task is likely to be using it.
- */
- if (preload_fpu)
- __math_state_restore();
-
return prev_p;
}

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index fc676e4..5afe824 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -571,28 +571,6 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
}

/*
- * __math_state_restore assumes that cr0.TS is already clear and the
- * fpu state is all ready for use. Used during context switch.
- */
-void __math_state_restore(void)
-{
- struct thread_info *thread = current_thread_info();
- struct task_struct *tsk = thread->task;
-
- /*
- * Paranoid restore. send a SIGSEGV if we fail to restore the state.
- */
- if (unlikely(restore_fpu_checking(tsk))) {
- stts();
- force_sig(SIGSEGV, tsk);
- return;
- }
-
- __thread_set_has_fpu(thread); /* clts in caller! */
- tsk->fpu_counter++;
-}
-
-/*
* 'math_state_restore()' saves the current math information in the
* old math state array, and gets the new ones from the current task
*
@@ -622,9 +600,18 @@ void math_state_restore(void)
local_irq_disable();
}

- clts(); /* Allow maths ops (or we recurse) */
+ __thread_fpu_begin(thread);

- __math_state_restore();
+ /*
+ * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+ */
+ if (unlikely(restore_fpu_checking(tsk))) {
+ __thread_fpu_end(thread);
+ force_sig(SIGSEGV, tsk);
+ return;
+ }
+
+ tsk->fpu_counter++;
}
EXPORT_SYMBOL_GPL(math_state_restore);

--
1.7.6.5

[-- Attachment #10: fpu-patch-82.diff --]
[-- Type: text/plain, Size: 4835 bytes --]

From 4903062b5485f0e2c286a23b44c9b59d9b017d53 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 16 Feb 2012 19:11:15 -0800
Subject: [PATCH] i387: move AMD K7/K8 fpu fxsave/fxrstor workaround from save
to restore

The AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception is
pending. In order to not leak FIP state from one process to another, we
need to do a floating point load after the fxsave of the old process,
and before the fxrstor of the new FPU state. That resets the state to
the (uninteresting) kernel load, rather than some potentially sensitive
user information.

We used to do this directly after the FPU state save, but that is
actually very inconvenient, since it

(a) corrupts what is potentially perfectly good FPU state that we might
want to lazy avoid restoring later and

(b) on x86-64 it resulted in a very annoying ordering constraint, where
"__unlazy_fpu()" in the task switch needs to be delayed until after
the DS segment has been reloaded just to get the new DS value.

Coupling it to the fxrstor instead of the fxsave automatically avoids
both of these issues, and also ensures that we only do it when actually
necessary (the FP state after a save may never actually get used). It's
simply a much more natural place for the leaked state cleanup.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 19 -------------------
arch/x86/kernel/process_64.c | 5 ++---
arch/x86/kernel/traps.c | 14 ++++++++++++++
3 files changed, 16 insertions(+), 22 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 86974c7..01b115d 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -211,15 +211,6 @@ static inline void fpu_fxsave(struct fpu *fpu)

#endif /* CONFIG_X86_64 */

-/* We need a safe address that is cheap to find and that is already
- in L1 during context switch. The best choices are unfortunately
- different for UP and SMP */
-#ifdef CONFIG_SMP
-#define safe_address (__per_cpu_offset[0])
-#else
-#define safe_address (kstat_cpu(0).cpustat.user)
-#endif
-
/*
* These must be called with preempt disabled
*/
@@ -243,16 +234,6 @@ static inline void fpu_save_init(struct fpu *fpu)

if (unlikely(fpu->state->fxsave.swd & X87_FSW_ES))
asm volatile("fnclex");
-
- /* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
- is pending. Clear the x87 state here by setting it to fixed
- values. safe_address is a random variable that should be in L1 */
- alternative_input(
- ASM_NOP8 ASM_NOP2,
- "emms\n\t" /* clear stack tags */
- "fildl %P[addr]", /* set F?P to defined value */
- X86_FEATURE_FXSAVE_LEAK,
- [addr] "m" (safe_address));
}

static inline void __save_init_fpu(struct task_struct *tsk)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 992b4e5..753e803 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -387,6 +387,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
struct tss_struct *tss = &per_cpu(init_tss, cpu);
unsigned fsindex, gsindex;

+ __unlazy_fpu(prev_p);
+
/*
* Reload esp0, LDT and the page table pointer:
*/
@@ -415,9 +417,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)

load_TLS(next, cpu);

- /* Must be after DS reload */
- __unlazy_fpu(prev_p);
-
/*
* Leave lazy mode, flushing any hypercalls made here.
* This must be done before restoring TLS segments so
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 5afe824..4d42300 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -585,6 +585,10 @@ void math_state_restore(void)
struct thread_info *thread = current_thread_info();
struct task_struct *tsk = thread->task;

+ /* We need a safe address that is cheap to find and that is already
+ in L1. We just brought in "thread->task", so use that */
+#define safe_address (thread->task)
+
if (!tsk_used_math(tsk)) {
local_irq_enable();
/*
@@ -602,6 +606,16 @@ void math_state_restore(void)

__thread_fpu_begin(thread);

+ /* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
+ is pending. Clear the x87 state here by setting it to fixed
+ values. safe_address is a random variable that should be in L1 */
+ alternative_input(
+ ASM_NOP8 ASM_NOP2,
+ "emms\n\t" /* clear stack tags */
+ "fildl %P[addr]", /* set F?P to defined value */
+ X86_FEATURE_FXSAVE_LEAK,
+ [addr] "m" (safe_address));
+
/*
* Paranoid restore. send a SIGSEGV if we fail to restore the state.
*/
--
1.7.6.5

[-- Attachment #11: fpu-patch-91.diff --]
[-- Type: text/plain, Size: 11818 bytes --]

From 34ddc81a230b15c0e345b6b253049db731499f7e Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 18 Feb 2012 12:56:35 -0800
Subject: [PATCH] i387: re-introduce FPU state preloading at context switch
time

After all the FPU state cleanups and finally finding the problem that
caused all our FPU save/restore problems, this re-introduces the
preloading of FPU state that was removed in commit b3b0870ef3ff ("i387:
do not preload FPU state at task switch time").

However, instead of simply reverting the removal, this reimplements
preloading with several fixes, most notably

- properly abstracted as a true FPU state switch, rather than as
open-coded save and restore with various hacks.

In particular, implementing it as a proper FPU state switch allows us
to optimize the CR0.TS flag accesses: there is no reason to set the
TS bit only to then almost immediately clear it again. CR0 accesses
are quite slow and expensive, don't flip the bit back and forth for
no good reason.

- Make sure that the same model works for both x86-32 and x86-64, so
that there are no gratuitous differences between the two due to the
way they save and restore segment state differently due to
architectural differences that really don't matter to the FPU state.

- Avoid exposing the "preload" state to the context switch routines,
and in particular allow the concept of lazy state restore: if nothing
else has used the FPU in the meantime, and the process is still on
the same CPU, we can avoid restoring state from memory entirely, just
re-expose the state that is still in the FPU unit.

That optimized lazy restore isn't actually implemented here, but the
infrastructure is set up for it. Of course, older CPU's that use
'fnsave' to save the state cannot take advantage of this, since the
state saving also trashes the state.

In other words, there is now an actual _design_ to the FPU state saving,
rather than just random historical baggage. Hopefully it's easier to
follow as a result.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
arch/x86/include/asm/i387.h | 110 +++++++++++++++++++++++++++++++++++-------
arch/x86/kernel/process_32.c | 5 ++-
arch/x86/kernel/process_64.c | 5 ++-
arch/x86/kernel/traps.c | 55 ++++++++++++---------
4 files changed, 133 insertions(+), 42 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index f537667..a850b4d 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -29,6 +29,7 @@ extern unsigned int sig_xstate_size;
extern void fpu_init(void);
extern void mxcsr_feature_mask_init(void);
extern int init_fpu(struct task_struct *child);
+extern void __math_state_restore(struct task_struct *);
extern void math_state_restore(void);
extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);

@@ -212,9 +213,10 @@ static inline void fpu_fxsave(struct fpu *fpu)
#endif /* CONFIG_X86_64 */

/*
- * These must be called with preempt disabled
+ * These must be called with preempt disabled. Returns
+ * 'true' if the FPU state is still intact.
*/
-static inline void fpu_save_init(struct fpu *fpu)
+static inline int fpu_save_init(struct fpu *fpu)
{
if (use_xsave()) {
fpu_xsave(fpu);
@@ -223,22 +225,33 @@ static inline void fpu_save_init(struct fpu *fpu)
* xsave header may indicate the init state of the FP.
*/
if (!(fpu->state->xsave.xsave_hdr.xstate_bv & XSTATE_FP))
- return;
+ return 1;
} else if (use_fxsr()) {
fpu_fxsave(fpu);
} else {
asm volatile("fnsave %[fx]; fwait"
: [fx] "=m" (fpu->state->fsave));
- return;
+ return 0;
}

- if (unlikely(fpu->state->fxsave.swd & X87_FSW_ES))
+ /*
+ * If exceptions are pending, we need to clear them so
+ * that we don't randomly get exceptions later.
+ *
+ * FIXME! Is this perhaps only true for the old-style
+ * irq13 case? Maybe we could leave the x87 state
+ * intact otherwise?
+ */
+ if (unlikely(fpu->state->fxsave.swd & X87_FSW_ES)) {
asm volatile("fnclex");
+ return 0;
+ }
+ return 1;
}

-static inline void __save_init_fpu(struct task_struct *tsk)
+static inline int __save_init_fpu(struct task_struct *tsk)
{
- fpu_save_init(&tsk->thread.fpu);
+ return fpu_save_init(&tsk->thread.fpu);
}

static inline int fpu_fxrstor_checking(struct fpu *fpu)
@@ -301,20 +314,79 @@ static inline void __thread_fpu_begin(struct task_struct *tsk)
}

/*
- * Signal frame handlers...
+ * FPU state switching for scheduling.
+ *
+ * This is a two-stage process:
+ *
+ * - switch_fpu_prepare() saves the old state and
+ * sets the new state of the CR0.TS bit. This is
+ * done within the context of the old process.
+ *
+ * - switch_fpu_finish() restores the new state as
+ * necessary.
*/
-extern int save_i387_xstate(void __user *buf);
-extern int restore_i387_xstate(void __user *buf);
+typedef struct { int preload; } fpu_switch_t;
+
+/*
+ * FIXME! We could do a totally lazy restore, but we need to
+ * add a per-cpu "this was the task that last touched the FPU
+ * on this CPU" variable, and the task needs to have a "I last
+ * touched the FPU on this CPU" and check them.
+ *
+ * We don't do that yet, so "fpu_lazy_restore()" always returns
+ * false, but some day..
+ */
+#define fpu_lazy_restore(tsk) (0)
+#define fpu_lazy_state_intact(tsk) do { } while (0)
+
+static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct task_struct *new)
+{
+ fpu_switch_t fpu;
+
+ fpu.preload = tsk_used_math(new) && new->fpu_counter > 5;
+ if (__thread_has_fpu(old)) {
+ if (__save_init_fpu(old))
+ fpu_lazy_state_intact(old);
+ __thread_clear_has_fpu(old);
+ old->fpu_counter++;
+
+ /* Don't change CR0.TS if we just switch! */
+ if (fpu.preload) {
+ __thread_set_has_fpu(new);
+ prefetch(new->thread.fpu.state);
+ } else
+ stts();
+ } else {
+ old->fpu_counter = 0;
+ if (fpu.preload) {
+ if (fpu_lazy_restore(new))
+ fpu.preload = 0;
+ else
+ prefetch(new->thread.fpu.state);
+ __thread_fpu_begin(new);
+ }
+ }
+ return fpu;
+}

-static inline void __unlazy_fpu(struct task_struct *tsk)
+/*
+ * By the time this gets called, we've already cleared CR0.TS and
+ * given the process the FPU if we are going to preload the FPU
+ * state - all we need to do is to conditionally restore the register
+ * state itself.
+ */
+static inline void switch_fpu_finish(struct task_struct *new, fpu_switch_t fpu)
{
- if (__thread_has_fpu(tsk)) {
- __save_init_fpu(tsk);
- __thread_fpu_end(tsk);
- } else
- tsk->fpu_counter = 0;
+ if (fpu.preload)
+ __math_state_restore(new);
}

+/*
+ * Signal frame handlers...
+ */
+extern int save_i387_xstate(void __user *buf);
+extern int restore_i387_xstate(void __user *buf);
+
static inline void __clear_fpu(struct task_struct *tsk)
{
if (__thread_has_fpu(tsk)) {
@@ -474,7 +546,11 @@ static inline void save_init_fpu(struct task_struct *tsk)
static inline void unlazy_fpu(struct task_struct *tsk)
{
preempt_disable();
- __unlazy_fpu(tsk);
+ if (__thread_has_fpu(tsk)) {
+ __save_init_fpu(tsk);
+ __thread_fpu_end(tsk);
+ } else
+ tsk->fpu_counter = 0;
preempt_enable();
}

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 324cd72..80bfe1a 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -299,10 +299,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
*next = &next_p->thread;
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
+ fpu_switch_t fpu;

/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */

- __unlazy_fpu(prev_p);
+ fpu = switch_fpu_prepare(prev_p, next_p);

/*
* Reload esp0.
@@ -357,6 +358,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
if (prev->gs | next->gs)
lazy_load_gs(next->gs);

+ switch_fpu_finish(next_p, fpu);
+
percpu_write(current_task, next_p);

return prev_p;
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 753e803..1fd94bc 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -386,8 +386,9 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
unsigned fsindex, gsindex;
+ fpu_switch_t fpu;

- __unlazy_fpu(prev_p);
+ fpu = switch_fpu_prepare(prev_p, next_p);

/*
* Reload esp0, LDT and the page table pointer:
@@ -457,6 +458,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
wrmsrl(MSR_KERNEL_GS_BASE, next->gs);
prev->gsindex = gsindex;

+ switch_fpu_finish(next_p, fpu);
+
/*
* Switch the PDA and FPU contexts.
*/
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index ad25e51..77da5b4 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -571,6 +571,37 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
}

/*
+ * This gets called with the process already owning the
+ * FPU state, and with CR0.TS cleared. It just needs to
+ * restore the FPU register state.
+ */
+void __math_state_restore(struct task_struct *tsk)
+{
+ /* We need a safe address that is cheap to find and that is already
+ in L1. We've just brought in "tsk->thread.has_fpu", so use that */
+#define safe_address (tsk->thread.has_fpu)
+
+ /* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
+ is pending. Clear the x87 state here by setting it to fixed
+ values. safe_address is a random variable that should be in L1 */
+ alternative_input(
+ ASM_NOP8 ASM_NOP2,
+ "emms\n\t" /* clear stack tags */
+ "fildl %P[addr]", /* set F?P to defined value */
+ X86_FEATURE_FXSAVE_LEAK,
+ [addr] "m" (safe_address));
+
+ /*
+ * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+ */
+ if (unlikely(restore_fpu_checking(tsk))) {
+ __thread_fpu_end(tsk);
+ force_sig(SIGSEGV, tsk);
+ return;
+ }
+}
+
+/*
* 'math_state_restore()' saves the current math information in the
* old math state array, and gets the new ones from the current task
*
@@ -584,10 +615,6 @@ void math_state_restore(void)
{
struct task_struct *tsk = current;

- /* We need a safe address that is cheap to find and that is already
- in L1. We're just bringing in "tsk->thread.has_fpu", so use that */
-#define safe_address (tsk->thread.has_fpu)
-
if (!tsk_used_math(tsk)) {
local_irq_enable();
/*
@@ -604,25 +631,7 @@ void math_state_restore(void)
}

__thread_fpu_begin(tsk);
-
- /* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
- is pending. Clear the x87 state here by setting it to fixed
- values. safe_address is a random variable that should be in L1 */
- alternative_input(
- ASM_NOP8 ASM_NOP2,
- "emms\n\t" /* clear stack tags */
- "fildl %P[addr]", /* set F?P to defined value */
- X86_FEATURE_FXSAVE_LEAK,
- [addr] "m" (safe_address));
-
- /*
- * Paranoid restore. send a SIGSEGV if we fail to restore the state.
- */
- if (unlikely(restore_fpu_checking(tsk))) {
- __thread_fpu_end(tsk);
- force_sig(SIGSEGV, tsk);
- return;
- }
+ __math_state_restore(tsk);

tsk->fpu_counter++;
}
--
1.7.6.5