[riscv] Hold virtual address offset in the thread pointer register

iPXE does not make use of any thread-local storage.  Use the otherwise
unused thread pointer register ("tp") to hold the current value of
the virtual address offset, rather than using a global variable.

This ensures that virt_offset can be made valid even during very early
initialisation (when iPXE may be executing directly from read-only
memory and so cannot update a global variable).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/src/arch/riscv/include/bits/virt_offset.h b/src/arch/riscv/include/bits/virt_offset.h
new file mode 100644
index 000000000..83ac17551
--- /dev/null
+++ b/src/arch/riscv/include/bits/virt_offset.h
@@ -0,0 +1,33 @@
+#ifndef _BITS_VIRT_OFFSET_H
+#define _BITS_VIRT_OFFSET_H
+
+/** @file
+ *
+ * RISCV-specific virtual address offset
+ *
+ * We use the thread pointer register (tp) to hold the virtual address
+ * offset, so that virtual-to-physical address translations work as
+ * expected even while we are executing directly from read-only memory
+ * (and so cannot store a value in a global virt_offset variable).
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/**
+ * Read virtual address offset held in thread pointer register
+ *
+ * @ret virt_offset	Virtual address offset
+ */
+static inline __attribute__ (( const, always_inline )) unsigned long
+tp_virt_offset ( void ) {
+	register unsigned long tp asm ( "tp" );
+
+	__asm__ ( "" : "=r" ( tp ) );
+	return tp;
+}
+
+/** Always read thread pointer register to get virtual address offset */
+#define virt_offset tp_virt_offset()
+
+#endif /* _BITS_VIRT_OFFSET_H */