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x86/fpu: Default eagerfpu=on on all CPUs

We have eager and lazy FPU modes, introduced in:

  304bced ("x86, fpu: use non-lazy fpu restore for processors supporting xsave")

The result is rather messy.  There are two code paths in almost all
of the FPU code, and only one of them (the eager case) is tested
frequently, since most kernel developers have new enough hardware
that we use eagerfpu.

It seems that, on any remotely recent hardware, eagerfpu is a win:
glibc uses SSE2, so laziness is probably overoptimistic, and, in any
case, manipulating TS is far slower that saving and restoring the
full state.  (Stores to CR0.TS are serializing and are poorly
optimized.)

To try to shake out any latent issues on old hardware, this changes
the default to eager on all CPUs.  If no performance or functionality
problems show up, a subsequent patch could remove lazy mode entirely.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: yu-cheng yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/ac290de61bf08d9cfc2664a4f5080257ffc1075a.1453675014.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Andy Lutomirski Ingo Molnar
Andy Lutomirski authored and Ingo Molnar committed Jan 24, 2016
1 parent c6ab109 commit 58122bf1d856a4ea9581d62a07c557d997d46a19
Showing with 5 additions and 8 deletions.
  1. +5 −8 arch/x86/kernel/fpu/init.c
@@ -260,7 +260,10 @@ static void __init fpu__init_system_xstate_size_legacy(void)
* not only saved the restores along the way, but we also have the
* FPU ready to be used for the original task.
*
* 'eager' switching is used on modern CPUs, there we switch the FPU
* 'lazy' is deprecated because it's almost never a performance win
* and it's much more complicated than 'eager'.
*
* 'eager' switching is by default on all CPUs, there we switch the FPU
* state during every context switch, regardless of whether the task
* has used FPU instructions in that time slice or not. This is done
* because modern FPU context saving instructions are able to optimize
@@ -271,7 +274,7 @@ static void __init fpu__init_system_xstate_size_legacy(void)
* to use 'eager' restores, if we detect that a task is using the FPU
* frequently. See the fpu->counter logic in fpu/internal.h for that. ]
*/
static enum { AUTO, ENABLE, DISABLE } eagerfpu = AUTO;
static enum { ENABLE, DISABLE } eagerfpu = ENABLE;
/*
* Find supported xfeatures based on cpu features and command-line input.
@@ -348,15 +351,9 @@ static void __init fpu__init_system_ctx_switch(void)
*/
static void __init fpu__init_parse_early_param(void)
{
/*
* No need to check "eagerfpu=auto" again, since it is the
* initial default.
*/
if (cmdline_find_option_bool(boot_command_line, "eagerfpu=off")) {
eagerfpu = DISABLE;
fpu__clear_eager_fpu_features();
} else if (cmdline_find_option_bool(boot_command_line, "eagerfpu=on")) {
eagerfpu = ENABLE;
}
if (cmdline_find_option_bool(boot_command_line, "no387"))

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