[riscv] Check if seed CSR is accessible from S-mode

The seed CSR defined by the Zkr extension is accessible only in M-mode
by default.  Older versions of OpenSBI (prior to version 1.4) do not
set mseccfg.sseed, with the result that attempts to access the seed
CSR from S-mode will raise an illegal instruction exception.

Add a facility for testing the accessibility of arbitrary CSRs, and
use it to check that the seed CSR is accessible before reporting the
seed CSR entropy source as being functional.

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

src/arch/riscv/core/zkr.c

@@ -31,6 +31,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <errno.h>
 #include <ipxe/hart.h>
+#include <ipxe/csr.h>
 #include <ipxe/entropy.h>
 #include <ipxe/drbg.h>
 
@@ -60,6 +61,12 @@ static int zkr_entropy_enable ( void ) {
 		return rc;
 	}
 
+	/* Check if seed CSR is accessible in S-mode */
+	if ( ! csr_can_write ( "seed", 0 ) ) {
+		DBGC ( colour, "ZKR cannot access seed CSR\n" );
+		return -ENOTSUP;
+	}
+
 	/* RISC-V ISA mandates that 128 bits of full entropy shall be
 	 * obtained from 256 entropy bits read from the seed CSR.
 	 *

src/arch/riscv/include/ipxe/csr.h

@@ -0,0 +1,75 @@
+#ifndef _IPXE_CSR_H
+#define _IPXE_CSR_H
+
+/** @file
+ *
+ * Control and status registers (CSRs)
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/**
+ * Check if CSR can be read
+ *
+ * @v name		CSR name
+ * @v allowed		CSR can be read
+ */
+#define csr_can_read( name ) ( {					\
+	unsigned long stvec_orig;					\
+	unsigned long stvec_temp;					\
+	unsigned long csr;						\
+	int allowed = 0;						\
+									\
+	__asm__ __volatile__ ( /* Set temporary trap vector */		\
+			       "la %3, 1f\n\t"				\
+			       "csrrw %2, stvec, %3\n\t"		\
+			       /* Try reading CSR */			\
+			       "csrr %1, " name "\n\t"			\
+			       /* Mark as allowed if not trapped */	\
+			       "addi %0, %0, 1\n\t"			\
+			       /* Temporary trap vector */		\
+			       ".balign 4\n\t"				\
+			       "\n1:\n\t"				\
+			       /* Restore original trap vector */	\
+			       "csrw stvec, %2\n\t"			\
+			       : "+r" ( allowed ),			\
+				 "=r" ( csr ),				\
+				 "=r" ( stvec_orig ),			\
+				 "=r" ( stvec_temp ) );			\
+	allowed;							\
+	} )
+
+/**
+ * Check if CSR can be written
+ *
+ * @v name		CSR name
+ * @v value		Value to write
+ * @v allowed		CSR can be written
+ */
+#define csr_can_write( name, value ) ( {				\
+	unsigned long stvec_orig;					\
+	unsigned long stvec_temp;					\
+	unsigned long csr = (value);					\
+	int allowed = 0;						\
+									\
+	__asm__ __volatile__ ( /* Set temporary trap vector */		\
+			       "la %3, 1f\n\t"				\
+			       "csrrw %2, stvec, %3\n\t"		\
+			       /* Try writing CSR */			\
+			       "csrrw %1, " name ", %1\n\t"		\
+			       /* Mark as allowed if not trapped */	\
+			       "addi %0, %0, 1\n\t"			\
+			       /* Temporary trap vector */		\
+			       ".balign 4\n\t"				\
+			       "\n1:\n\t"				\
+			       /* Restore original trap vector */	\
+			       "csrw stvec, %2\n\t"			\
+			       : "+r" ( allowed ),			\
+				 "+r" ( csr ),				\
+				 "=r" ( stvec_orig ),			\
+				 "=r" ( stvec_temp ) );			\
+	allowed;							\
+	} )
+
+#endif /* _IPXE_CSR_H */