Commit 62b95a7b authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Russell King (Oracle)
Browse files

ARM: 9282/1: vfp: Manipulate task VFP state with softirqs disabled 



In a subsequent patch, we will relax the kernel mode NEON policy, and
permit kernel mode NEON to be used not only from task context, as is
permitted today, but also from softirq context.

Given that softirqs may trigger over the back of any IRQ unless they are
explicitly disabled, we need to address the resulting races in the VFP
state handling, by disabling softirq processing in two distinct but
related cases:
- kernel mode NEON will leave the FPU disabled after it completes, so
  any kernel code sequence that enables the FPU and subsequently accesses
  its registers needs to disable softirqs until it completes;
- kernel_neon_begin() will preserve the userland VFP state in memory,
  and if it interrupts the ordinary VFP state preserve sequence, the
  latter will resume execution with the VFP registers corrupted, and
  happily continue saving them to memory.

Given that disabling softirqs also disables preemption, we can replace
the existing preempt_disable/enable occurrences in the VFP state
handling asm code with new macros that dis/enable softirqs instead.
In the VFP state handling C code, add local_bh_disable/enable() calls
in those places where the VFP state is preserved.

One thing to keep in mind is that, once we allow NEON use in softirq
context, the result of any such interruption is that the FPEXC_EN bit in
the FPEXC register will be cleared, and vfp_current_hw_state[cpu] will
be NULL. This means that any sequence that [conditionally] clears
FPEXC_EN and/or sets vfp_current_hw_state[cpu] to NULL does not need to
run with softirqs disabled, as the result will be the same. Furthermore,
the handling of THREAD_NOTIFY_SWITCH is guaranteed to run with IRQs
disabled, and so it does not need protection from softirq interruptions
either.

Tested-by: default avatarMartin Willi <martin@strongswan.org>
Reviewed-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
Signed-off-by: default avatarRussell King (Oracle) <rmk+kernel@armlinux.org.uk>
parent 368ccecd

arch/arm/include/asm/assembler.h

+12 −7
Original line number Diff line number Diff line
@@ -236,21 +236,26 @@ THUMB( fpreg .req r7 ) 
	sub	\tmp, \tmp, #1			@ decrement it

	str	\tmp, [\ti, #TI_PREEMPT]

	.endm



	.macro	dec_preempt_count_ti, ti, tmp

	get_thread_info \ti

	dec_preempt_count \ti, \tmp

	.endm

#else

	.macro	inc_preempt_count, ti, tmp

	.endm



	.macro	dec_preempt_count, ti, tmp

	.endm

#endif



	.macro	local_bh_disable, ti, tmp

	ldr	\tmp, [\ti, #TI_PREEMPT]

	add	\tmp, \tmp, #SOFTIRQ_DISABLE_OFFSET

	str	\tmp, [\ti, #TI_PREEMPT]

	.endm



	.macro	dec_preempt_count_ti, ti, tmp

	.macro	local_bh_enable_ti, ti, tmp

	get_thread_info \ti

	ldr	\tmp, [\ti, #TI_PREEMPT]

	sub	\tmp, \tmp, #SOFTIRQ_DISABLE_OFFSET

	str	\tmp, [\ti, #TI_PREEMPT]

	.endm

#endif



#define USERL(l, x...)				\

9999:	x;					\

arch/arm/kernel/asm-offsets.c

+1 −0
Original line number Diff line number Diff line
@@ -56,6 +56,7 @@ int main(void) 
  DEFINE(VFP_CPU,		offsetof(union vfp_state, hard.cpu));

#endif

#endif

  DEFINE(SOFTIRQ_DISABLE_OFFSET,SOFTIRQ_DISABLE_OFFSET);

#ifdef CONFIG_ARM_THUMBEE

  DEFINE(TI_THUMBEE_STATE,	offsetof(struct thread_info, thumbee_state));

#endif

arch/arm/vfp/entry.S

+2 −2
Original line number Diff line number Diff line
@@ -22,7 +22,7 @@ 
@  IRQs enabled.

@

ENTRY(do_vfp)

	inc_preempt_count r10, r4

	local_bh_disable r10, r4

 	ldr	r4, .LCvfp

	ldr	r11, [r10, #TI_CPU]	@ CPU number

	add	r10, r10, #TI_VFPSTATE	@ r10 = workspace
@@ -30,7 +30,7 @@ ENTRY(do_vfp) 
ENDPROC(do_vfp)



ENTRY(vfp_null_entry)

	dec_preempt_count_ti r10, r4

	local_bh_enable_ti r10, r4

	ret	lr

ENDPROC(vfp_null_entry)

arch/arm/vfp/vfphw.S

+2 −2
Original line number Diff line number Diff line
@@ -175,7 +175,7 @@ vfp_hw_state_valid: 
					@ else it's one 32-bit instruction, so

					@ always subtract 4 from the following

					@ instruction address.

	dec_preempt_count_ti r10, r4

	local_bh_enable_ti r10, r4

	ret	r9			@ we think we have handled things
@@ -200,7 +200,7 @@ skip: 
	@ not recognised by VFP



	DBGSTR	"not VFP"

	dec_preempt_count_ti r10, r4

	local_bh_enable_ti r10, r4

	ret	lr



process_exception:

arch/arm/vfp/vfpmodule.c

+7 −1
Original line number Diff line number Diff line
@@ -416,7 +416,7 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs) 
	if (exceptions)

		vfp_raise_exceptions(exceptions, trigger, orig_fpscr, regs);

 exit:

	preempt_enable();

	local_bh_enable();

}



static void vfp_enable(void *unused)
@@ -517,6 +517,8 @@ void vfp_sync_hwstate(struct thread_info *thread) 
{

	unsigned int cpu = get_cpu();



	local_bh_disable();



	if (vfp_state_in_hw(cpu, thread)) {

		u32 fpexc = fmrx(FPEXC);
@@ -528,6 +530,7 @@ void vfp_sync_hwstate(struct thread_info *thread) 
		fmxr(FPEXC, fpexc);

	}



	local_bh_enable();

	put_cpu();

}
@@ -717,6 +720,8 @@ void kernel_neon_begin(void) 
	unsigned int cpu;

	u32 fpexc;



	local_bh_disable();



	/*

	 * Kernel mode NEON is only allowed outside of interrupt context

	 * with preemption disabled. This will make sure that the kernel
@@ -739,6 +744,7 @@ void kernel_neon_begin(void) 
		vfp_save_state(vfp_current_hw_state[cpu], fpexc);

#endif

	vfp_current_hw_state[cpu] = NULL;

	local_bh_enable();

}

EXPORT_SYMBOL(kernel_neon_begin);