Re: [PATCH v3 04/17] riscv: misaligned: request misaligned exception from SBI

[Andrew Jones <ajones@ventanamicro.com>]

On Mon, Mar 10, 2025 at 04:12:11PM +0100, Clément Léger wrote:
> Now that the kernel can handle misaligned accesses in S-mode, request
> misaligned access exception delegation from SBI. This uses the FWFT SBI
> extension defined in SBI version 3.0.
> 
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> ---
>  arch/riscv/include/asm/cpufeature.h        |  3 +-
>  arch/riscv/kernel/traps_misaligned.c       | 77 +++++++++++++++++++++-
>  arch/riscv/kernel/unaligned_access_speed.c | 11 +++-
>  3 files changed, 86 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
> index 569140d6e639..ad7d26788e6a 100644
> --- a/arch/riscv/include/asm/cpufeature.h
> +++ b/arch/riscv/include/asm/cpufeature.h
> @@ -64,8 +64,9 @@ void __init riscv_user_isa_enable(void);
>  	_RISCV_ISA_EXT_DATA(_name, _id, _sub_exts, ARRAY_SIZE(_sub_exts), _validate)
>  
>  bool check_unaligned_access_emulated_all_cpus(void);
> +void unaligned_access_init(void);
> +int cpu_online_unaligned_access_init(unsigned int cpu);
>  #if defined(CONFIG_RISCV_SCALAR_MISALIGNED)
> -void check_unaligned_access_emulated(struct work_struct *work __always_unused);
>  void unaligned_emulation_finish(void);
>  bool unaligned_ctl_available(void);
>  DECLARE_PER_CPU(long, misaligned_access_speed);
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index 7cc108aed74e..90ac74191357 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -16,6 +16,7 @@
>  #include <asm/entry-common.h>
>  #include <asm/hwprobe.h>
>  #include <asm/cpufeature.h>
> +#include <asm/sbi.h>
>  #include <asm/vector.h>
>  
>  #define INSN_MATCH_LB			0x3
> @@ -635,7 +636,7 @@ bool check_vector_unaligned_access_emulated_all_cpus(void)
>  
>  static bool unaligned_ctl __read_mostly;
>  
> -void check_unaligned_access_emulated(struct work_struct *work __always_unused)
> +static void check_unaligned_access_emulated(struct work_struct *work __always_unused)
>  {
>  	int cpu = smp_processor_id();
>  	long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
> @@ -646,6 +647,13 @@ void check_unaligned_access_emulated(struct work_struct *work __always_unused)
>  	__asm__ __volatile__ (
>  		"       "REG_L" %[tmp], 1(%[ptr])\n"
>  		: [tmp] "=r" (tmp_val) : [ptr] "r" (&tmp_var) : "memory");
> +}
> +
> +static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
> +{
> +	long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
> +
> +	check_unaligned_access_emulated(NULL);
>  
>  	/*
>  	 * If unaligned_ctl is already set, this means that we detected that all
> @@ -654,9 +662,10 @@ void check_unaligned_access_emulated(struct work_struct *work __always_unused)
>  	 */
>  	if (unlikely(unaligned_ctl && (*mas_ptr != RISCV_HWPROBE_MISALIGNED_SCALAR_EMULATED))) {
>  		pr_crit("CPU misaligned accesses non homogeneous (expected all emulated)\n");
> -		while (true)
> -			cpu_relax();
> +		return -EINVAL;
>  	}
> +
> +	return 0;
>  }
>  
>  bool check_unaligned_access_emulated_all_cpus(void)
> @@ -688,4 +697,66 @@ bool check_unaligned_access_emulated_all_cpus(void)
>  {
>  	return false;
>  }
> +static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
> +{
> +	return 0;
> +}
>  #endif
> +
> +#ifdef CONFIG_RISCV_SBI
> +
> +static bool misaligned_traps_delegated;
> +
> +static int cpu_online_sbi_unaligned_setup(unsigned int cpu)
> +{
> +	if (sbi_fwft_set(SBI_FWFT_MISALIGNED_EXC_DELEG, 1, 0) &&
> +	    misaligned_traps_delegated) {
> +		pr_crit("Misaligned trap delegation non homogeneous (expected delegated)");
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +static void unaligned_sbi_request_delegation(void)
> +{
> +	int ret;
> +
> +	ret = sbi_fwft_all_cpus_set(SBI_FWFT_MISALIGNED_EXC_DELEG, 1, 0, 0);
> +	if (ret)
> +		return;
> +
> +	misaligned_traps_delegated = true;
> +	pr_info("SBI misaligned access exception delegation ok\n");
> +	/*
> +	 * Note that we don't have to take any specific action here, if
> +	 * the delegation is successful, then
> +	 * check_unaligned_access_emulated() will verify that indeed the
> +	 * platform traps on misaligned accesses.
> +	 */
> +}
> +
> +void unaligned_access_init(void)
> +{
> +	if (sbi_probe_extension(SBI_EXT_FWFT) > 0)
> +		unaligned_sbi_request_delegation();
> +}
> +#else
> +void unaligned_access_init(void) {}
> +
> +static int cpu_online_sbi_unaligned_setup(unsigned int cpu __always_unused)
> +{
> +	return 0;
> +}
> +#endif
> +
> +int cpu_online_unaligned_access_init(unsigned int cpu)
> +{
> +	int ret;
> +
> +	ret = cpu_online_sbi_unaligned_setup(cpu);
> +	if (ret)
> +		return ret;
> +
> +	return cpu_online_check_unaligned_access_emulated(cpu);
> +}
> diff --git a/arch/riscv/kernel/unaligned_access_speed.c b/arch/riscv/kernel/unaligned_access_speed.c
> index 91f189cf1611..2f3aba073297 100644
> --- a/arch/riscv/kernel/unaligned_access_speed.c
> +++ b/arch/riscv/kernel/unaligned_access_speed.c
> @@ -188,13 +188,20 @@ arch_initcall_sync(lock_and_set_unaligned_access_static_branch);
>  
>  static int riscv_online_cpu(unsigned int cpu)
>  {
> +	int ret;
>  	static struct page *buf;
>  
>  	/* We are already set since the last check */
>  	if (per_cpu(misaligned_access_speed, cpu) != RISCV_HWPROBE_MISALIGNED_SCALAR_UNKNOWN)
>  		goto exit;
>  
> -	check_unaligned_access_emulated(NULL);
> +	ret = cpu_online_unaligned_access_init(cpu);
> +	if (ret)
> +		return ret;
> +
> +	if (per_cpu(misaligned_access_speed, cpu) == RISCV_HWPROBE_MISALIGNED_SCALAR_EMULATED)
> +		goto exit;
> +
>  	buf = alloc_pages(GFP_KERNEL, MISALIGNED_BUFFER_ORDER);
>  	if (!buf) {
>  		pr_warn("Allocation failure, not measuring misaligned performance\n");
> @@ -403,6 +410,8 @@ static int check_unaligned_access_all_cpus(void)
>  {
>  	bool all_cpus_emulated, all_cpus_vec_unsupported;
>  
> +	unaligned_access_init();
> +
>  	all_cpus_emulated = check_unaligned_access_emulated_all_cpus();
>  	all_cpus_vec_unsupported = check_vector_unaligned_access_emulated_all_cpus();
>  
> -- 
> 2.47.2
>

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>