[tip:x86/urgent] x86/fpu: Revert ("x86/fpu: Disable AVX when eagerfpu is off")

From: tip-bot for Yu-cheng Yu
Date: Thu Mar 10 2016 - 06:06:21 EST

Commit-ID: a65050c6f17e52442716138d48d0a47301a8344b
Gitweb: http://git.kernel.org/tip/a65050c6f17e52442716138d48d0a47301a8344b
Author: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
AuthorDate: Wed, 9 Mar 2016 16:28:54 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 10 Mar 2016 10:15:58 +0100

x86/fpu: Revert ("x86/fpu: Disable AVX when eagerfpu is off")

Leonid Shatz noticed that the SDM interpretation of the following
recent commit:

394db20ca240741 ("x86/fpu: Disable AVX when eagerfpu is off")

... is incorrect and that the original behavior of the FPU code was correct.

Because AVX is not stated in CR0 TS bit description, it was mistakenly
believed to be not supported for lazy context switch. This turns out
to be false:

Intel Software Developer's Manual Vol. 3A, Sec. 2.5 Control Registers:

'TS Task Switched bit (bit 3 of CR0) -- Allows the saving of the x87 FPU/
MMX/SSE/SSE2/SSE3/SSSE3/SSE4 context on a task switch to be delayed until
an x87 FPU/MMX/SSE/SSE2/SSE3/SSSE3/SSE4 instruction is actually executed
by the new task.'

Intel Software Developer's Manual Vol. 2A, Sec. 2.4 Instruction Exception

'AVX instructions refer to exceptions by classes that include #NM
"Device Not Available" exception for lazy context switch.'

So revert the commit.

Reported-by: Leonid Shatz <leonid.shatz@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1457569734-3785-1-git-send-email-yu-cheng.yu@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
arch/x86/include/asm/fpu/xstate.h | 9 ++++-----
arch/x86/kernel/fpu/init.c | 6 ------
2 files changed, 4 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index af30fde..f23cd8c 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -20,16 +20,15 @@

/* Supported features which support lazy state saving */
-/* Supported features which require eager state saving */

+/* Supported features which require eager state saving */
/* All currently supported features */

diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index d53ab3d..9ee7e30 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -302,12 +302,6 @@ u64 __init fpu__get_supported_xfeatures_mask(void)
static void __init fpu__clear_eager_fpu_features(void)
- setup_clear_cpu_cap(X86_FEATURE_AVX);
- setup_clear_cpu_cap(X86_FEATURE_AVX2);
- setup_clear_cpu_cap(X86_FEATURE_AVX512F);
- setup_clear_cpu_cap(X86_FEATURE_AVX512PF);
- setup_clear_cpu_cap(X86_FEATURE_AVX512ER);
- setup_clear_cpu_cap(X86_FEATURE_AVX512CD);