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x86/fpu: Add more comments to the FPU init code 

Extend the comments of the FPU init code, and fix old ones.

Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
hifive-unleashed-5.1
Ingo Molnar 2015-04-26 15:36:46 +02:00
parent 41e78410d8
commit ae02679c56
1 changed files with 58 additions and 12 deletions
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70
arch/x86/kernel/fpu/init.c
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@ -1,9 +1,13 @@
/*
* x86 FPU boot time init code
* x86 FPU boot time init code:
*/
#include <asm/fpu/internal.h>
#include <asm/tlbflush.h>
/*
* Initialize the TS bit in CR0 according to the style of context-switches
* we are using:
*/
static void fpu__init_cpu_ctx_switch(void)
{
if (!cpu_has_eager_fpu)
@ -35,7 +39,7 @@ static void fpu__init_cpu_generic(void)
}
/*
* Enable all supported FPU features. Called when a CPU is brought online.
* Enable all supported FPU features. Called when a CPU is brought online:
*/
void fpu__init_cpu(void)
{
@ -71,8 +75,7 @@ static void fpu__init_system_early_generic(struct cpuinfo_x86 *c)
#ifndef CONFIG_MATH_EMULATION
if (!cpu_has_fpu) {
pr_emerg("No FPU found and no math emulation present\n");
pr_emerg("Giving up\n");
pr_emerg("x86/fpu: Giving up, no FPU found and no math emulation present\n");
for (;;)
asm volatile("hlt");
}
@ -120,6 +123,12 @@ static void fpu__init_system_generic(void)
fpu__init_system_mxcsr();
}
/*
* Size of the FPU context state. All tasks in the system use the
* same context size, regardless of what portion they use.
* This is inherent to the XSAVE architecture which puts all state
* components into a single, continuous memory block:
*/
unsigned int xstate_size;
EXPORT_SYMBOL_GPL(xstate_size);
@ -158,6 +167,37 @@ static void fpu__init_system_xstate_size_legacy(void)
}
}
/*
* FPU context switching strategies:
*
* Against popular belief, we don't do lazy FPU saves, due to the
* task migration complications it brings on SMP - we only do
* lazy FPU restores.
*
* 'lazy' is the traditional strategy, which is based on setting
* CR0::TS to 1 during context-switch (instead of doing a full
* restore of the FPU state), which causes the first FPU instruction
* after the context switch (whenever it is executed) to fault - at
* which point we lazily restore the FPU state into FPU registers.
*
* Tasks are of course under no obligation to execute FPU instructions,
* so it can easily happen that another context-switch occurs without
* a single FPU instruction being executed. If we eventually switch
* back to the original task (that still owns the FPU) then we have
* 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
* 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
* state saving and restoration in hardware: they can detect both
* unused and untouched FPU state and optimize accordingly.
*
* [ Note that even in 'lazy' mode we might optimize context switches
* 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 int __init eager_fpu_setup(char *s)
@ -173,8 +213,7 @@ static int __init eager_fpu_setup(char *s)
__setup("eagerfpu=", eager_fpu_setup);
/*
* setup_init_fpu_buf() is __init and it is OK to call it here because
* init_xstate_ctx will be unset only once during boot.
* Pick the FPU context switching strategy:
*/
static void fpu__init_system_ctx_switch(void)
{
@ -202,20 +241,24 @@ static void fpu__init_system_ctx_switch(void)
}
/*
* Called on the boot CPU once per system bootup, to set up the initial FPU state that
* is later cloned into all processes.
* Called on the boot CPU once per system bootup, to set up the initial
* FPU state that is later cloned into all processes:
*/
void fpu__init_system(struct cpuinfo_x86 *c)
{
fpu__init_system_early_generic(c);
/* The FPU has to be operational for some of the later FPU init activities: */
/*
* The FPU has to be operational for some of the
* later FPU init activities:
*/
fpu__init_cpu();
/*
* But don't leave CR0::TS set yet, as some of the FPU setup methods depend
* on being able to execute FPU instructions that will fault on a set TS,
* such as the FXSAVE in fpu__init_system_mxcsr().
* But don't leave CR0::TS set yet, as some of the FPU setup
* methods depend on being able to execute FPU instructions
* that will fault on a set TS, such as the FXSAVE in
* fpu__init_system_mxcsr().
*/
clts();
@ -226,6 +269,9 @@ void fpu__init_system(struct cpuinfo_x86 *c)
fpu__init_system_ctx_switch();
}
/*
* Boot parameter to turn off FPU support and fall back to math-emu:
*/
static int __init no_387(char *s)
{
setup_clear_cpu_cap(X86_FEATURE_FPU);