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Commit ae02679c authored by Ingo Molnar's avatar Ingo Molnar
Browse files

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



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

Reviewed-by: default avatarBorislav 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: default avatarIngo Molnar <mingo@kernel.org>
parent 41e78410

arch/x86/kernel/fpu/init.c

+58 −12
Original line number Diff line number Diff line 
/*

 * 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);