linux-kernel - Re: [PATCH v2 2/3] arch/arm64: fpsimd: Introduce fpsimd_save_and_flush_cpu

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Message-ID: <20190416115305.GL3567@e103592.cambridge.arm.com>
Date:   Tue, 16 Apr 2019 12:53:06 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        ard.biesheuvel@...aro.org, julien.thierry@....com,
        marc.zyngier@....com, catalin.marinas@....com,
        suzuki.poulose@....com, will.deacon@....com,
        christoffer.dall@....com, james.morse@....com
Subject: Re: [PATCH v2 2/3] arch/arm64: fpsimd: Introduce
 fpsimd_save_and_flush_cpu_state() and use it

On Fri, Apr 12, 2019 at 06:14:19PM +0100, Julien Grall wrote:
> The only external user of fpsimd_save() and fpsimd_flush_cpu_state() is
> the KVM FPSIMD code.
> 
> A follow-up patch will mandate hold of the FPSIMD context while

This is a bit hard to read without understanding the following patch.

Maybe elaborate a bit with something like

"The following patch will introduce a mechanism to acquire ownership of the FPSIMD/SVE context for performing context management operations. Rather than having to export this [...]"

> modifying it. Rather than having to export the new helpers to get/put
> the context, we can just introduce a new function to combine
> fpsimd_save() and fpsimd_flush_cpu_state().
> 
> This has also the advantage to remove any external call of fpsimd_save()
> and fpsimd_flush_cpu_state(), so they can be turned static.
> 
> Lastly, the new function can also be used in the PM notifier.
> 
> Signed-off-by: Julien Grall <julien.grall@....com>
> 
> ---
>     Changes in v2:
>         - Patch added
> ---
>  arch/arm64/include/asm/fpsimd.h |  5 ++---
>  arch/arm64/kernel/fpsimd.c      | 17 +++++++++++++----
>  arch/arm64/kvm/fpsimd.c         |  3 +--
>  3 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> index 32abbfd852e8..88c223f6ded6 100644
> --- a/arch/arm64/include/asm/fpsimd.h
> +++ b/arch/arm64/include/asm/fpsimd.h
> @@ -48,11 +48,11 @@ struct task_struct;
>  extern void fpsimd_save_state(struct user_fpsimd_state *state);
>  extern void fpsimd_load_state(struct user_fpsimd_state *state);
>  
> -extern void fpsimd_save(void);
> -
>  extern void fpsimd_thread_switch(struct task_struct *next);
>  extern void fpsimd_flush_thread(void);
>  
> +extern void fpsimd_save_and_flush_cpu_state(void);
> +

Nit, bit I think this makes more sense alongside the other flush
functions.

Can you just drop this new declaration in place of the old declaration
for sve_flush_cpu_state()?

>  extern void fpsimd_signal_preserve_current_state(void);
>  extern void fpsimd_preserve_current_state(void);
>  extern void fpsimd_restore_current_state(void);
> @@ -63,7 +63,6 @@ extern void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *state,
>  				     void *sve_state, unsigned int sve_vl);
>  
>  extern void fpsimd_flush_task_state(struct task_struct *target);
> -extern void fpsimd_flush_cpu_state(void);
>  
>  /* Maximum VL that SVE VL-agnostic software can transparently support */
>  #define SVE_VL_ARCH_MAX 0x100
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 577296bba730..9e4e4b6acd93 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -241,7 +241,7 @@ static void task_fpsimd_load(void)
>   *
>   * Softirqs (and preemption) must be disabled.
>   */
> -void fpsimd_save(void)
> +static void fpsimd_save(void)
>  {
>  	struct fpsimd_last_state_struct const *last =
>  		this_cpu_ptr(&fpsimd_last_state);
> @@ -1117,12 +1117,22 @@ void fpsimd_flush_task_state(struct task_struct *t)
>   * Invalidate any task's FPSIMD state that is present on this cpu.
>   * This function must be called with softirqs disabled.
>   */
> -void fpsimd_flush_cpu_state(void)
> +static void fpsimd_flush_cpu_state(void)
>  {
>  	__this_cpu_write(fpsimd_last_state.st, NULL);
>  	set_thread_flag(TIF_FOREIGN_FPSTATE);
>  }
>  
> +/*
> + * Save the FPSIMD state to memory and invalidate cpu view.
> + * This function must be called with softirqs (and preemption) disabled.
> + */
> +void fpsimd_save_and_flush_cpu_state(void)
> +{
> +	fpsimd_save();
> +	fpsimd_flush_cpu_state();
> +}
> +
>  #ifdef CONFIG_KERNEL_MODE_NEON
>  
>  DEFINE_PER_CPU(bool, kernel_neon_busy);
> @@ -1279,8 +1289,7 @@ static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
>  {
>  	switch (cmd) {
>  	case CPU_PM_ENTER:
> -		fpsimd_save();
> -		fpsimd_flush_cpu_state();
> +		fpsimd_save_and_flush_cpu_state();

Any reason not to do this in kernel_neon_begin() too?

>  		break;
>  	case CPU_PM_EXIT:
>  		break;
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index 6e3c9c8b2df9..01bd2e862506 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -113,8 +113,7 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
>  		u64 *guest_zcr = &vcpu->arch.ctxt.sys_regs[ZCR_EL1];
>  
>  		/* Clean guest FP state to memory and invalidate cpu view */

You can lose the comment here, since the call is now self-explanatory.

> -		fpsimd_save();
> -		fpsimd_flush_cpu_state();
> +		fpsimd_save_and_flush_cpu_state();

[...]

Otherwise, looks sensible.

Cheers
---Dave