riscv: vector: allow kernel-mode Vector with preemption

Add kernel_vstate to keep track of kernel-mode Vector registers when
trap introduced context switch happens. Also, provide riscv_v_flags to
let context save/restore routine track context status. Context tracking
happens whenever the core starts its in-kernel Vector executions. An
active (dirty) kernel task's V contexts will be saved to memory whenever
a trap-introduced context switch happens. Or, when a softirq, which
happens to nest on top of it, uses Vector. Context retoring happens when
the execution transfer back to the original Kernel context where it
first enable preempt_v.

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 of 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
switch overhead.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20240115055929.4736-11-andy.chiu@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 25 insertions, 6 deletions

diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index f7b4aeb9e457..6727d1d3b8f2 100644
--- a/arch/riscv/kernel/vector.c
+++ b/arch/riscv/kernel/vector.c
@@ -22,6 +22,9 @@
 
 static bool riscv_v_implicit_uacc = IS_ENABLED(CONFIG_RISCV_ISA_V_DEFAULT_ENABLE);
 static struct kmem_cache *riscv_v_user_cachep;
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+static struct kmem_cache *riscv_v_kernel_cachep;
+#endif
 
 unsigned long riscv_v_vsize __read_mostly;
 EXPORT_SYMBOL_GPL(riscv_v_vsize);
@@ -56,6 +59,11 @@ void __init riscv_v_setup_ctx_cache(void)
 	riscv_v_user_cachep = kmem_cache_create_usercopy("riscv_vector_ctx",
 							 riscv_v_vsize, 16, SLAB_PANIC,
 							 0, riscv_v_vsize, NULL);
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+	riscv_v_kernel_cachep = kmem_cache_create("riscv_vector_kctx",
+						  riscv_v_vsize, 16,
+						  SLAB_PANIC, NULL);
+#endif
 }
 
 static bool insn_is_vector(u32 insn_buf)
@@ -91,24 +99,35 @@ static bool insn_is_vector(u32 insn_buf)
 	return false;
 }
 
-static int riscv_v_thread_zalloc(void)
+static int riscv_v_thread_zalloc(struct kmem_cache *cache,
+				 struct __riscv_v_ext_state *ctx)
 {
 	void *datap;
 
-	datap = kmem_cache_zalloc(riscv_v_user_cachep, GFP_KERNEL);
+	datap = kmem_cache_zalloc(cache, GFP_KERNEL);
 	if (!datap)
 		return -ENOMEM;
 
-	current->thread.vstate.datap = datap;
-	memset(&current->thread.vstate, 0, offsetof(struct __riscv_v_ext_state,
-						    datap));
+	ctx->datap = datap;
+	memset(ctx, 0, offsetof(struct __riscv_v_ext_state, datap));
 	return 0;
 }
 
+void riscv_v_thread_alloc(struct task_struct *tsk)
+{
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+	riscv_v_thread_zalloc(riscv_v_kernel_cachep, &tsk->thread.kernel_vstate);
+#endif
+}
+
 void riscv_v_thread_free(struct task_struct *tsk)
 {
 	if (tsk->thread.vstate.datap)
 		kmem_cache_free(riscv_v_user_cachep, tsk->thread.vstate.datap);
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+	if (tsk->thread.kernel_vstate.datap)
+		kmem_cache_free(riscv_v_kernel_cachep, tsk->thread.kernel_vstate.datap);
+#endif
 }
 
 #define VSTATE_CTRL_GET_CUR(x) ((x) & PR_RISCV_V_VSTATE_CTRL_CUR_MASK)
@@ -180,7 +199,7 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
 	 * context where VS has been off. So, try to allocate the user's V
 	 * context and resume execution.
 	 */
-	if (riscv_v_thread_zalloc()) {
+	if (riscv_v_thread_zalloc(riscv_v_user_cachep, &current->thread.vstate)) {
 		force_sig(SIGBUS);
 		return true;
 	}