[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>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/arch/riscv/core/zkr.c b/src/arch/riscv/core/zkr.c
index bf1609671..4b6316ea0 100644
--- a/src/arch/riscv/core/zkr.c
+++ b/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.
 	 *