Re: [v2, 5/5] riscv: vector: allow kernel-mode Vector with preemption

Conor Dooley
2023-07-24T12:19:12Z

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Hey Andy,

On Fri, Jul 21, 2023 at 11:28:55AM +0000, Andy Chiu wrote:
> Add kernel_vstate to keep track of kernel-mode Vector registers when
> trap introduced context switch happens. Also, provide trap_pt_regs to
> let context save/restore routine reference status.VS at which the trap
> takes place. The thread flag TIF_RISCV_V_KERNEL_MODE indicates whether
> a task is running in kernel-mode Vector with preemption 'ON'. So context
> switch routines know and would save V-regs to kernel_vstate and restore
> V-regs immediately from kernel_vstate if the bit is set.
>
> Apart from a task's preemption status, the capability of
> running preemptive kernel-mode Vector is jointly controlled by the
> RISCV_V_VSTATE_CTRL_PREEMPTIBLE mask in the task's
> thread.vstate_ctrl. This bit is masked whenever a trap takes place in
> kernel mode while executing preemptive Vector code.
>
> Also, provide a config CONFIG_RISCV_ISA_V_PREEMPTIVE to give users an
> option to disable preemptible kernel-mode Vector at build time. Users
> with constraint memory may want to disable this config as preemptible
> kernel-mode Vector needs extra space for tracking per thread's
> kernel-mode V context. Or, users might as well want to disable it if all
> kernel-mode Vector code is time sensitive and cannot tolerate context
> swicth overhead.
>
> Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> ---
> Changelog v2:
> - fix build fail when compiling without RISCV_ISA_V (Conor)
> - 's/TIF_RISCV_V_KMV/TIF_RISCV_V_KERNEL_MODE' and add comment (Conor)
> - merge Kconfig patch into this oine (Conor).
> - 's/CONFIG_RISCV_ISA_V_PREEMPTIVE_KMV/CONFIG_RISCV_ISA_V_PREEMPTIVE/'
> (Conor)
> - fix some typos (Conor)
> - enclose assembly with RISCV_ISA_V_PREEMPTIVE.
> - change riscv_v_vstate_ctrl_config_kmv() to
> kernel_vector_allow_preemption() for better understanding. (Conor)
> - 's/riscv_v_kmv_preempitble/kernel_vector_preemptible/'
> ---
> arch/riscv/Kconfig | 10 +++++
> arch/riscv/include/asm/processor.h | 2 +
> arch/riscv/include/asm/simd.h | 4 +-
> arch/riscv/include/asm/thread_info.h | 4 ++
> arch/riscv/include/asm/vector.h | 27 +++++++++++--
> arch/riscv/kernel/asm-offsets.c | 2 +
> arch/riscv/kernel/entry.S | 45 ++++++++++++++++++++++
> arch/riscv/kernel/kernel_mode_vector.c | 53 ++++++++++++++++++++++++--
> arch/riscv/kernel/process.c | 8 +++-
> arch/riscv/kernel/vector.c | 3 +-
> 10 files changed, 148 insertions(+), 10 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 4c07b9189c86..0622951b15dd 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -507,6 +507,16 @@ config RISCV_ISA_V_DEFAULT_ENABLE
>
> If you don't know what to do here, say Y.
>
> +config RISCV_ISA_V_PREEMPTIVE
> + bool "Run kernel-mode Vector with kernel preemption"
> + depends on PREEMPTION
> + depends on RISCV_ISA_V
> + default y
> + help
> + Ordinarily the kernel disables preemption before running in-kernel
> + Vector code. This config frees the kernel from disabling preemption
> + by adding memory on demand for tracking kernel's V-context.
> +
> config TOOLCHAIN_HAS_ZBB
> bool
> default y
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index c950a8d9edef..497c0dd30b2a 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -42,6 +42,8 @@ struct thread_struct {
> unsigned long bad_cause;
> unsigned long vstate_ctrl;
> struct __riscv_v_ext_state vstate;
> + struct pt_regs *trap_pt_regs;
> + struct __riscv_v_ext_state kernel_vstate;
> };
>
> /* Whitelist the fstate from the task_struct for hardened usercopy */
> diff --git a/arch/riscv/include/asm/simd.h b/arch/riscv/include/asm/simd.h
> index ef70af78005d..a54a0ce58f4d 100644
> --- a/arch/riscv/include/asm/simd.h
> +++ b/arch/riscv/include/asm/simd.h
> @@ -12,6 +12,7 @@
> #include <linux/percpu.h>
> #include <linux/preempt.h>
> #include <linux/types.h>
> +#include <linux/thread_info.h>
>
> #ifdef CONFIG_RISCV_ISA_V
>
> @@ -35,7 +36,8 @@ static __must_check inline bool may_use_simd(void)
> * where it is set.
> */
> return !in_irq() && !irqs_disabled() && !in_nmi() &&
> - !this_cpu_read(vector_context_busy);
> + !this_cpu_read(vector_context_busy) &&
> + !test_thread_flag(TIF_RISCV_V_KERNEL_MODE);
> }
>
> #else /* ! CONFIG_RISCV_ISA_V */
> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> index b182f2d03e25..8797d520e8ef 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -94,6 +94,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
> #define TIF_UPROBE 10 /* uprobe breakpoint or singlestep */
> #define TIF_32BIT 11 /* compat-mode 32bit process */
> #define TIF_RISCV_V_DEFER_RESTORE 12 /* restore Vector before returing to user */
> +#define TIF_RISCV_V_KERNEL_MODE 13 /* kernel-mode Vector run with preemption-on */
>
> #define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
> #define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
> @@ -101,9 +102,12 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
> #define _TIF_NOTIFY_SIGNAL (1 << TIF_NOTIFY_SIGNAL)
> #define _TIF_UPROBE (1 << TIF_UPROBE)
> #define _TIF_RISCV_V_DEFER_RESTORE (1 << TIF_RISCV_V_DEFER_RESTORE)
> +#define _TIF_RISCV_V_KERNEL_MODE (1 << TIF_RISCV_V_KERNEL_MODE)
>
> #define _TIF_WORK_MASK \
> (_TIF_NOTIFY_RESUME | _TIF_SIGPENDING | _TIF_NEED_RESCHED | \
> _TIF_NOTIFY_SIGNAL | _TIF_UPROBE)
>
> +#define RISCV_V_VSTATE_CTRL_PREEMPTIBLE 0x20
> +
> #endif /* _ASM_RISCV_THREAD_INFO_H */
> diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> index 3b783b317112..c2776851d50d 100644
> --- a/arch/riscv/include/asm/vector.h
> +++ b/arch/riscv/include/asm/vector.h
> @@ -195,9 +195,24 @@ static inline void __switch_to_vector(struct task_struct *prev,
> {
> struct pt_regs *regs;
>
> - regs = task_pt_regs(prev);
> - riscv_v_vstate_save(&prev->thread.vstate, regs);
> - riscv_v_vstate_set_restore(next, task_pt_regs(next));
> + if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE) &&
> + test_tsk_thread_flag(prev, TIF_RISCV_V_KERNEL_MODE)) {
> + regs = prev->thread.trap_pt_regs;
> + WARN_ON(!regs);

In what cases could these WARN_ON()s be triggered?

> + riscv_v_vstate_save(&prev->thread.kernel_vstate, regs);
> + } else {
> + regs = task_pt_regs(prev);
> + riscv_v_vstate_save(&prev->thread.vstate, regs);
> + }
> +
> + if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE) &&
> + test_tsk_thread_flag(next, TIF_RISCV_V_KERNEL_MODE)) {
> + regs = next->thread.trap_pt_regs;
> + WARN_ON(!regs);
> + riscv_v_vstate_restore(&next->thread.kernel_vstate, regs);
> + } else {
> + riscv_v_vstate_set_restore(next, task_pt_regs(next));
> + }
> }

> /*
> * kernel_vector_begin(): obtain the CPU vector registers for use by the calling
> * context
> @@ -70,11 +109,14 @@ void kernel_vector_begin(void)
>
> riscv_v_vstate_save(&current->thread.vstate, task_pt_regs(current));
>
> - get_cpu_vector_context();
> + if (!preemptible() || !kernel_vector_preemptible()) {
> + get_cpu_vector_context();
> + } else {
> + if (riscv_v_start_kernel_context())
> + get_cpu_vector_context();

What happens here if riscv_v_start_kernel_context() fails w/ -ENOMEM?

> + }
>
> riscv_v_enable();
> -
> - return 0;
> }
> EXPORT_SYMBOL_GPL(kernel_vector_begin);
>
> @@ -96,6 +138,9 @@ void kernel_vector_end(void)
>
> riscv_v_disable();
>
> - put_cpu_vector_context();
> + if (!test_thread_flag(TIF_RISCV_V_KERNEL_MODE))
> + put_cpu_vector_context();
> + else
> + riscv_v_stop_kernel_context();
> }

Probably just missing something here, but how come we don't need to call
put_cpu_vector_context() here. I'm just a little confused, since, in
kernel_vector_begin, get_cpu_vector_context() is called.

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