Re: [PATCH v2 1/2] arm64/sme: Restore SMCR on exit from suspend

[Will Deacon]

On Sat, Feb 03, 2024 at 01:00:40PM +0000, Mark Brown wrote:
> The fields in SMCR_EL1 reset to an architecturally UNKNOWN value. Since we
> do not otherwise manage the traps configured in this register at runtime we
> need to reconfigure them after a suspend in case nothing else was kind
> enough to preserve them for us.
> 
> The vector length will be restored as part of restoring the SME state for
> the next SME using task.
> 
> Fixes: a1f4ccd25cc2 (arm64/sme: Provide Kconfig for SME)
> Reported-by: Jackson Cooper-Driver <Jackson.Cooper-Driver@xxxxxxx>
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/fpsimd.h |  2 ++
>  arch/arm64/kernel/fpsimd.c      | 14 ++++++++++++++
>  arch/arm64/kernel/suspend.c     |  3 +++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> index 50e5f25d3024..7780d343ef08 100644
> --- a/arch/arm64/include/asm/fpsimd.h
> +++ b/arch/arm64/include/asm/fpsimd.h
> @@ -386,6 +386,7 @@ extern void sme_alloc(struct task_struct *task, bool flush);
>  extern unsigned int sme_get_vl(void);
>  extern int sme_set_current_vl(unsigned long arg);
>  extern int sme_get_current_vl(void);
> +extern void sme_suspend_exit(void);
>  
>  /*
>   * Return how many bytes of memory are required to store the full SME
> @@ -421,6 +422,7 @@ static inline int sme_max_vl(void) { return 0; }
>  static inline int sme_max_virtualisable_vl(void) { return 0; }
>  static inline int sme_set_current_vl(unsigned long arg) { return -EINVAL; }
>  static inline int sme_get_current_vl(void) { return -EINVAL; }
> +static inline void sme_suspend_exit(void) { }
>  
>  static inline size_t sme_state_size(struct task_struct const *task)
>  {
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index a5dc6f764195..8d2a5824d5d3 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -1311,6 +1311,20 @@ void __init sme_setup(void)
>  		get_sme_default_vl());
>  }
>  
> +void sme_suspend_exit(void)
> +{
> +	u64 smcr = 0;
> +
> +	if (!system_supports_sme())
> +		return;
> +
> +	if (system_supports_fa64())
> +		smcr |= SMCR_ELx_FA64;
> +
> +	write_sysreg_s(smcr, SYS_SMCR_EL1);
> +	write_sysreg_s(0, SYS_SMPRI_EL1);
> +}

Looking at the other places where we touch SMCR_EL1, it looks like we
always use a read-modify-write sequence. However, doesn't that mean we
inherit a bunch of unknown bits on cold boot? I'm basically wondering
whether we should be initialising these registers to a well-known value
earlier in the CPU init path, a bit like we do for the EL2 variants.

Will