[PATCH] lib: sbi_illegal_insn: Emulate c.li

[Inochi Amaoto]

>The Linux kernel RISC-V image format allows that UEFI Images can also
>be booted by non-UEFI firmware. However for that to work, the PE/Image
>combo requires that 'MZ' is a valid instruction. On RISC-V, 'MZ' is
>only a valid instruction if the hardware is C capable [1]. So add
>Emulate c.li
>
>Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
>Signed-off-by: Xiang W <wxjstz at 126.com>
>---
> lib/sbi/sbi_illegal_insn.c | 18 +++++++++++++++++-
> 1 file changed, 17 insertions(+), 1 deletion(-)
>
>diff --git a/lib/sbi/sbi_illegal_insn.c b/lib/sbi/sbi_illegal_insn.c
>index 2be4757..4ab10f4 100644
>--- a/lib/sbi/sbi_illegal_insn.c
>+++ b/lib/sbi/sbi_illegal_insn.c
>@@ -102,6 +102,22 @@ static int system_opcode_insn(ulong insn, struct sbi_trap_regs *regs)
> 	return 0;
> }
>
>+static int compressed_insn(ulong insn, struct sbi_trap_regs *regs) {
>+	unsigned long imm, rd;
>+	unsigned long *regs_p = (unsigned long *)regs;
>+
>+	if ((insn & 0xe003) == 0x4001) { /* c.li */
>+		imm = (insn >> 2) & 0x1f;
>+		imm |= ((insn >> 12) & 1) ? -32 : 0;
>+		rd = (insn >> 7) & 0x1f;
>+		if (rd)
>+			regs_p[rd] = imm;
>+		return 0;
>+	}
>+
>+	return truly_illegal_insn(insn, regs);
>+}
>+
> static const illegal_insn_func illegal_insn_table[32] = {
> 	truly_illegal_insn, /* 0 */
> 	truly_illegal_insn, /* 1 */
>@@ -160,7 +176,7 @@ int sbi_illegal_insn_handler(ulong insn, struct sbi_trap_regs *regs)
> 			return sbi_trap_redirect(regs, &uptrap);
> 		}
> 		if ((insn & 3) != 3)
>-			return truly_illegal_insn(insn, regs);
>+			return compressed_insn(insn, regs);
> 	}
>
> 	return illegal_insn_table[(insn & 0x7c) >> 2](insn, regs);
>--
>2.42.0
>

I think this patch may be unnecessary.

After searching the boot flow of UEFI, I only found two implementations:

For QEMU RISC-V virt platform and SG2042 platform:
SBI -> (SEC -> PEI -> DXE) -> bootloader -> OS
               UEFI

For SiFive U540 platform:
(SEC -> OpenSBI -> PEI -> DXE) -> bootloader -> OS
             UEFI

Both invoke SBI at a very early stage and use raw vaild address, and
handling the UEFI header is more like bootloader's job. I have no idea
about why this should be handled by SBI. FYI, Björn Töpel.