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

Andy Chiu
2023-07-21T11:30:07Z

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);
+ 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));
+ }
}

void riscv_v_vstate_ctrl_init(struct task_struct *tsk);
@@ -222,4 +237,10 @@ static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }

#endif /* CONFIG_RISCV_ISA_V */

+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+void kernel_vector_allow_preemption(void);
+#else
+#define kernel_vector_allow_preemption() do {} while (0)
+#endif
+
#endif /* ! __ASM_RISCV_VECTOR_H */
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index d6a75aac1d27..4b062f7741b2 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -38,6 +38,8 @@ void asm_offsets(void)
OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
+ OFFSET(TASK_THREAD_TRAP_REGP, task_struct, thread.trap_pt_regs);
+ OFFSET(TASK_THREAD_VSTATE_CTRL, task_struct, thread.vstate_ctrl);

OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
OFFSET(TASK_THREAD_F1, task_struct, thread.fstate.f[1]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 143a2bb3e697..b6a7d4e9f526 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -66,6 +66,29 @@ _save_context:
REG_S s4, PT_CAUSE(sp)
REG_S s5, PT_TP(sp)

+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+ /*
+ * Record the register set at the frame where in-kernel V registers are
+ * last alive.
+ */
+ REG_L s0, TASK_TI_FLAGS(tp)
+ li s1, 1 << TIF_RISCV_V_KERNEL_MODE
+ and s0, s0, s1
+ beqz s0, 1f
+ li s0, TASK_THREAD_TRAP_REGP
+ add s0, s0, tp
+ REG_L s1, (s0)
+ bnez s1, 1f
+ REG_S sp, (s0)
+ li s0, TASK_THREAD_VSTATE_CTRL
+ add s0, s0, tp
+ REG_L s1, (s0)
+ li s2, ~RISCV_V_VSTATE_CTRL_PREEMPTIBLE
+ and s1, s1, s2
+ REG_S s1, (s0)
+1:
+#endif
+
/*
* Set the scratch register to 0, so that if a recursive exception
* occurs, the exception vector knows it came from the kernel
@@ -129,6 +152,28 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
*/
csrw CSR_SCRATCH, tp
1:
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+ /*
+ * Clear tracking of the trap registers when we return to the frame
+ * that uses kernel mode Vector.
+ */
+ REG_L s0, TASK_TI_FLAGS(tp)
+ li s1, 1 << TIF_RISCV_V_KERNEL_MODE
+ and s0, s0, s1
+ beqz s0, 1f
+ li s0, TASK_THREAD_TRAP_REGP
+ add s0, s0, tp
+ REG_L s1, (s0)
+ bne s1, sp, 1f
+ REG_S x0, (s0)
+ li s0, TASK_THREAD_VSTATE_CTRL
+ add s0, s0, tp
+ REG_L s1, (s0)
+ ori s1, s1, RISCV_V_VSTATE_CTRL_PREEMPTIBLE
+ REG_S s1, (s0)
+1:
+#endif
+
REG_L a0, PT_STATUS(sp)
/*
* The current load reservation is effectively part of the processor's
diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
index d9e097e68937..5c64f2034cdc 100644
--- a/arch/riscv/kernel/kernel_mode_vector.c
+++ b/arch/riscv/kernel/kernel_mode_vector.c
@@ -10,6 +10,7 @@
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/types.h>
+#include <linux/slab.h>

#include <asm/vector.h>
#include <asm/switch_to.h>
@@ -48,6 +49,44 @@ static void put_cpu_vector_context(void)
preempt_enable();
}

+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+void kernel_vector_allow_preemption(void)
+{
+ current->thread.vstate_ctrl |= RISCV_V_VSTATE_CTRL_PREEMPTIBLE;
+}
+
+static bool kernel_vector_preemptible(void)
+{
+ return !!(current->thread.vstate_ctrl & RISCV_V_VSTATE_CTRL_PREEMPTIBLE);
+}
+
+static int riscv_v_start_kernel_context(void)
+{
+ struct __riscv_v_ext_state *vstate;
+
+ vstate = &current->thread.kernel_vstate;
+ if (!vstate->datap) {
+ vstate->datap = kmalloc(riscv_v_vsize, GFP_KERNEL);
+ if (!vstate->datap)
+ return -ENOMEM;
+ }
+
+ current->thread.trap_pt_regs = NULL;
+ WARN_ON(test_and_set_thread_flag(TIF_RISCV_V_KERNEL_MODE));
+ return 0;
+}
+
+static void riscv_v_stop_kernel_context(void)
+{
+ WARN_ON(!test_and_clear_thread_flag(TIF_RISCV_V_KERNEL_MODE));
+ current->thread.trap_pt_regs = NULL;
+}
+#else
+#define kernel_vector_preemptible() (false)
+#define riscv_v_start_kernel_context() (0)
+#define riscv_v_stop_kernel_context() do {} while (0)
+#endif /* CONFIG_RISCV_ISA_V_PREEMPTIVE */
+
/*
* 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();
+ }

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();
}
EXPORT_SYMBOL_GPL(kernel_vector_end);
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index ec89e7edb6fd..18cb37c305ab 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -160,8 +160,11 @@ void flush_thread(void)
void arch_release_task_struct(struct task_struct *tsk)
{
/* Free the vector context of datap. */
- if (has_vector())
+ if (has_vector()) {
kfree(tsk->thread.vstate.datap);
+ if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE))
+ kfree(tsk->thread.kernel_vstate.datap);
+ }
}

int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
@@ -170,7 +173,9 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
*dst = *src;
/* clear entire V context, including datap for a new task */
memset(&dst->thread.vstate, 0, sizeof(struct __riscv_v_ext_state));
+ memset(&dst->thread.kernel_vstate, 0, sizeof(struct __riscv_v_ext_state));
clear_tsk_thread_flag(dst, TIF_RISCV_V_DEFER_RESTORE);
+ clear_tsk_thread_flag(dst, TIF_RISCV_V_KERNEL_MODE);

return 0;
}
@@ -205,6 +210,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
childregs->a0 = 0; /* Return value of fork() */
p->thread.s[0] = 0;
}
+ kernel_vector_allow_preemption();
p->thread.ra = (unsigned long)ret_from_fork;
p->thread.sp = (unsigned long)childregs; /* kernel sp */
return 0;
diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index 9d583b760db4..42f227077ee5 100644
--- a/arch/riscv/kernel/vector.c
+++ b/arch/riscv/kernel/vector.c
@@ -122,7 +122,8 @@ static inline void riscv_v_ctrl_set(struct task_struct *tsk, int cur, int nxt,
ctrl |= VSTATE_CTRL_MAKE_NEXT(nxt);
if (inherit)
ctrl |= PR_RISCV_V_VSTATE_CTRL_INHERIT;
- tsk->thread.vstate_ctrl = ctrl;
+ tsk->thread.vstate_ctrl &= ~PR_RISCV_V_VSTATE_CTRL_MASK;
+ tsk->thread.vstate_ctrl |= ctrl;
}

bool riscv_v_vstate_ctrl_user_allowed(void)
--
2.17.1