Patchwork [3.8.y.z,extended,stable] Patch "x86: Allow FPU to be used at interrupt time even with eagerfpu" has been added to staging queue

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Submitter Kamal Mostafa
Date June 4, 2013, 3:50 p.m.
Message ID <1370361037-5929-1-git-send-email-kamal@canonical.com>
Download mbox | patch
Permalink /patch/248695/
State New
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Comments

Kamal Mostafa - June 4, 2013, 3:50 p.m.
This is a note to let you know that I have just added a patch titled

    x86: Allow FPU to be used at interrupt time even with eagerfpu

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

From 7fb5176686bd8d28b5cc123850f9624508a66d8d Mon Sep 17 00:00:00 2001
From: Pekka Riikonen <priikone@iki.fi>
Date: Mon, 13 May 2013 14:32:07 +0200
Subject: x86: Allow FPU to be used at interrupt time even with eagerfpu

commit 5187b28ff08249ab8a162e802209ed04e271ca02 upstream.

With the addition of eagerfpu the irq_fpu_usable() now returns false
negatives especially in the case of ksoftirqd and interrupted idle task,
two common cases for FPU use for example in networking/crypto.  With
eagerfpu=off FPU use is possible in those contexts.  This is because of
the eagerfpu check in interrupted_kernel_fpu_idle():

...
  * For now, with eagerfpu we will return interrupted kernel FPU
  * state as not-idle. TBD: Ideally we can change the return value
  * to something like __thread_has_fpu(current). But we need to
  * be careful of doing __thread_clear_has_fpu() before saving
  * the FPU etc for supporting nested uses etc. For now, take
  * the simple route!
...
 	if (use_eager_fpu())
 		return 0;

As eagerfpu is automatically "on" on those CPUs that also have the
features like AES-NI this patch changes the eagerfpu check to return 1 in
case the kernel_fpu_begin() has not been said yet.  Once it has been the
__thread_has_fpu() will start returning 0.

Notice that with eagerfpu the __thread_has_fpu is always true initially.
FPU use is thus always possible no matter what task is under us, unless
the state has already been saved with kernel_fpu_begin().

[ hpa: this is a performance regression, not a correctness regression,
  but since it can be quite serious on CPUs which need encryption at
  interrupt time I am marking this for urgent/stable. ]

Signed-off-by: Pekka Riikonen <priikone@iki.fi>
Link: http://lkml.kernel.org/r/alpine.GSO.2.00.1305131356320.18@git.silcnet.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
 arch/x86/kernel/i387.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

--
1.8.1.2

Patch

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 245a71d..cb33909 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -22,23 +22,19 @@ 
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
- * For now, with eagerfpu we will return interrupted kernel FPU
- * state as not-idle. TBD: Ideally we can change the return value
- * to something like __thread_has_fpu(current). But we need to
- * be careful of doing __thread_clear_has_fpu() before saving
- * the FPU etc for supporting nested uses etc. For now, take
- * the simple route!
- *
  * On others, we can do a kernel_fpu_begin/end() pair *ONLY* if that
  * 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).
+ *
+ * Except for the eagerfpu case when we return 1 unless we've already
+ * been eager and saved the state in kernel_fpu_begin().
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
 	if (use_eager_fpu())
-		return 0;
+		return __thread_has_fpu(current);

 	return !__thread_has_fpu(current) &&
 		(read_cr0() & X86_CR0_TS);
@@ -78,8 +74,8 @@  void __kernel_fpu_begin(void)
 	struct task_struct *me = current;

 	if (__thread_has_fpu(me)) {
-		__save_init_fpu(me);
 		__thread_clear_has_fpu(me);
+		__save_init_fpu(me);
 		/* We do 'stts()' in __kernel_fpu_end() */
 	} else if (!use_eager_fpu()) {
 		this_cpu_write(fpu_owner_task, NULL);