Re: [v1, 2/6] riscv: Add support for kernel mode vector

[Conor Dooley <conor.dooley@microchip.com>]

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On Sat, Jul 15, 2023 at 03:00:28PM +0000, Andy Chiu wrote:
> From: Greentime Hu <greentime.hu@sifive.com>
> 
> Add kernel_rvv_begin() and kernel_rvv_end() function declarations
> and corresponding definitions in kernel_mode_vector.c
> 
> These are needed to wrap uses of vector in kernel mode.
> 
> Co-developed-by: Vincent Chen <vincent.chen@sifive.com>
> Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
> Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
> Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> ---
>  arch/riscv/include/asm/vector.h        |   2 +
>  arch/riscv/kernel/Makefile             |   1 +
>  arch/riscv/kernel/kernel_mode_vector.c | 129 +++++++++++++++++++++++++
>  3 files changed, 132 insertions(+)
>  create mode 100644 arch/riscv/kernel/kernel_mode_vector.c
> 
> diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> index a4f3705fd144..9831b19153ae 100644
> --- a/arch/riscv/include/asm/vector.h
> +++ b/arch/riscv/include/asm/vector.h
> @@ -22,6 +22,8 @@
>  extern unsigned long riscv_v_vsize;
>  int riscv_v_setup_vsize(void);
>  bool riscv_v_first_use_handler(struct pt_regs *regs);
> +int kernel_rvv_begin(void);
> +void kernel_rvv_end(void);

So, we ditched all of the "rvv" stuff in the last series, using either
"vector" - has_vector() - or "riscv_v". I'd rather not introduce a third
naming scheme for vector related things...

Given what you add below is full of other things that use "vector", how
does s/rvv/vector/ sound here?


> diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
> new file mode 100644
> index 000000000000..c0c152c501a5
> --- /dev/null
> +++ b/arch/riscv/kernel/kernel_mode_vector.c

> +/*
> + * kernel_rvv_begin(): obtain the CPU vector registers for use by the calling
> + * context
> + *
> + * Must not be called unless may_use_vector() returns true.
> + * Task context in the vector registers is saved back to memory as necessary.
> + *
> + * A matching call to kernel_rvv_end() must be made before returning from the
> + * calling context.
> + *
> + * The caller may freely use the vector registers until kernel_rvv_end() is
> + * called.
> + */
> +int kernel_rvv_begin(void)

How come this returns an int, but you never actually check the result? The
other kernel_*_begin()s don't seem to return anything other than void.

> +{
> +	if (!has_vector())
> +		return -EOPNOTSUPP;
> +
> +	if (!may_use_vector())
> +		return -EPERM;

I notice arm64 takes a stronger approach. There, if the has() call
fails, it has a WARN_ON(). For the !may_use() case, it BUG()s.
Since users are forced to check may_use_vector() before calling
kernel_rvv_begin().

Is there a reason that we should not take the same approach?

> +
> +	/* Save vector state, if any */
> +	riscv_v_vstate_save(current, task_pt_regs(current));
> +
> +	/* Acquire kernel mode vector */
> +	get_cpu_vector_context();
> +
> +	/* Enable vector */

These three comments are mostly a statement of the obvious, no?

> +	riscv_v_enable();
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(kernel_rvv_begin);
> +
> +/*
> + * kernel_rvv_end(): give the CPU vector registers back to the current task
> + *
> + * Must be called from a context in which kernel_rvv_begin() was previously
> + * called, with no call to kernel_rvv_end() in the meantime.
> + *
> + * The caller must not use the vector registers after this function is called,
> + * unless kernel_rvv_begin() is called again in the meantime.
> + */
> +void kernel_rvv_end(void)
> +{
> +	if (WARN_ON(!has_vector()))

But there is a WARN_ON() here...

> +		return;
> +
> +	/* Restore vector state, if any */
> +	riscv_v_vstate_set_restore(current, task_pt_regs(current));
> +
> +	/* disable vector */
> +	riscv_v_disable();
> +
> +	/* release kernel mode vector */

Again, comments kinda state the obvious, no?

Otherwise, this stuff looks generally fine to me & similar to what is
being done elsewhere.

Thanks,
Conor.

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