xen/riscv: disable fpu

Disable FPU to detect illegal usage of floating point in kernel
space.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@gmail.com>

xen/arch/riscv/riscv64/head.S

@@ -6,6 +6,13 @@ ENTRY(start)
         /* Mask all interrupts */
         csrw    CSR_SIE, zero
 
+        /*
+         * Disable FPU to detect illegal usage of
+         * floating point in kernel space
+         */
+        li      t0, SSTATUS_FS
+        csrc    CSR_SSTATUS, t0
+
         la      sp, cpu0_boot_stack
         li      t0, STACK_SIZE
         add     sp, sp, t0