riscv: Add support for kernel-mode FPU

This is needed to support recent hardware in the amdgpu DRM driver. The
FPU code in that driver is not performance-critical, so only provide the
minimal support.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Icenowy Zheng <uwu@icenowy.me>

arch/riscv/include/asm/switch_to.h

@@ -61,6 +61,20 @@ static __always_inline bool has_fpu(void)
 {
 	return static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_FPU]);
 }
+
+static inline void kernel_fpu_begin(void)
+{
+	preempt_disable();
+	fstate_save(current, task_pt_regs(current));
+	csr_set(CSR_SSTATUS, SR_FS);
+}
+
+static inline void kernel_fpu_end(void)
+{
+	csr_clear(CSR_SSTATUS, SR_FS);
+	fstate_restore(current, task_pt_regs(current));
+	preempt_enable();
+}
 #else
 static __always_inline bool has_fpu(void) { return false; }
 #define fstate_save(task, regs) do { } while (0)

arch/riscv/kernel/process.c

@@ -195,3 +195,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	p->thread.sp = (unsigned long)childregs; /* kernel sp */
 	return 0;
 }
+
+EXPORT_SYMBOL_GPL(__fstate_save);
+EXPORT_SYMBOL_GPL(__fstate_restore);