Commit 3931261e authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Will Deacon
Browse files

arm64: fpsimd: Bring cond_yield asm macro in line with new rules 



We no longer disable softirqs or preemption when doing kernel mode SIMD,
and so for fully preemptible kernels, there is no longer a need to do any
explicit yielding (and for non-preemptible kernels, yielding is not
needed either).

That leaves voluntary preemption, where only explicit yield calls may
result in a reschedule. To retain the existing behavior for such a
configuration, we should take the new situation into account, where the
preempt count will be zero rather than one, and yielding to pending
softirqs is unnecessary.

Fixes: aefbab8e ("arm64: fpsimd: Preserve/restore kernel mode NEON at context switch")
Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240111112447.577640-2-ardb+git@google.com


Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent 8c5a19cb

arch/arm64/include/asm/assembler.h

+9 −16
Original line number Diff line number Diff line
@@ -760,32 +760,25 @@ alternative_endif 
.endm



	/*

	 * Check whether preempt/bh-disabled asm code should yield as soon as

	 * it is able. This is the case if we are currently running in task

	 * context, and either a softirq is pending, or the TIF_NEED_RESCHED

	 * flag is set and re-enabling preemption a single time would result in

	 * a preempt count of zero. (Note that the TIF_NEED_RESCHED flag is

	 * stored negated in the top word of the thread_info::preempt_count

	 * Check whether asm code should yield as soon as it is able. This is

	 * the case if we are currently running in task context, and the

	 * TIF_NEED_RESCHED flag is set. (Note that the TIF_NEED_RESCHED flag

	 * is stored negated in the top word of the thread_info::preempt_count

	 * field)

	 */

	.macro		cond_yield, lbl:req, tmp:req, tmp2:req

	.macro		cond_yield, lbl:req, tmp:req, tmp2

#ifdef CONFIG_PREEMPT_VOLUNTARY

	get_current_task \tmp

	ldr		\tmp, [\tmp, #TSK_TI_PREEMPT]

	/*

	 * If we are serving a softirq, there is no point in yielding: the

	 * softirq will not be preempted no matter what we do, so we should

	 * run to completion as quickly as we can.

	 * run to completion as quickly as we can. The preempt_count field will

	 * have BIT(SOFTIRQ_SHIFT) set in this case, so the zero check will

	 * catch this case too.

	 */

	tbnz		\tmp, #SOFTIRQ_SHIFT, .Lnoyield_\@

#ifdef CONFIG_PREEMPTION

	sub		\tmp, \tmp, #PREEMPT_DISABLE_OFFSET

	cbz		\tmp, \lbl

#endif

	adr_l		\tmp, irq_stat + IRQ_CPUSTAT_SOFTIRQ_PENDING

	get_this_cpu_offset	\tmp2

	ldr		w\tmp, [\tmp, \tmp2]

	cbnz		w\tmp, \lbl	// yield on pending softirq in task context

.Lnoyield_\@:

	.endm



/*

arch/arm64/kernel/asm-offsets.c

+0 −2
Original line number Diff line number Diff line
@@ -117,8 +117,6 @@ int main(void) 
  DEFINE(DMA_FROM_DEVICE,	DMA_FROM_DEVICE);

  BLANK();

  DEFINE(PREEMPT_DISABLE_OFFSET, PREEMPT_DISABLE_OFFSET);

  DEFINE(SOFTIRQ_SHIFT, SOFTIRQ_SHIFT);

  DEFINE(IRQ_CPUSTAT_SOFTIRQ_PENDING, offsetof(irq_cpustat_t, __softirq_pending));

  BLANK();

  DEFINE(CPU_BOOT_TASK,		offsetof(struct secondary_data, task));

  BLANK();