riscv/stand: pass boot hart in loader metadata

Use the RISCV_EFI_BOOT_PROTOCOL to fetch the boot hart ID, and
communicate this to the kernel via new metadata field
(MODINFOMD_BOOT_HART).

If the boot hart is not found this way, fall back to the (now
deprecated) device-tree method.

The assumption that a hart ID can be represented with a 32-bit unsigned
integer is unchanged in the kernel, but from the loader we pass the full
64-bit value. This ensures that this API won't need to change in the
future, should the wider value become necessary.

Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D48887

stand/efi/loader/arch/riscv/exec.c

@@ -2,6 +2,7 @@
  * Copyright (c) 2001 Benno Rice <benno@FreeBSD.org>
  * Copyright (c) 2007 Semihalf, Rafal Jaworowski <raj@semihalf.com>
  * All rights reserved.
+ * Copyright (c) 2024 The FreeBSD Foundation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -40,6 +41,28 @@
 #include "bootstrap.h"
 #include "loader_efi.h"
 
+static void
+riscv_set_boot_hart(struct preloaded_file *fp)
+{
+	EFI_GUID riscvboot = RISCV_EFI_BOOT_PROTOCOL_GUID;
+	RISCV_EFI_BOOT_PROTOCOL *proto;
+	EFI_STATUS status = 0;
+	uint64_t boot_hartid = ULONG_MAX;
+
+	status = BS->LocateProtocol(&riscvboot, NULL, (void **)&proto);
+	if (EFI_ERROR(status)) {
+		return;
+	}
+
+	status = proto->GetBootHartId(proto, &boot_hartid);
+	if (EFI_ERROR(status)) {
+		return;
+	}
+
+	file_addmetadata(fp, MODINFOMD_BOOT_HARTID, sizeof(boot_hartid),
+	    &boot_hartid);
+}
+
 static int
 __elfN(exec)(struct preloaded_file *fp)
 {
@@ -52,6 +75,8 @@ __elfN(exec)(struct preloaded_file *fp)
 	if ((fmp = file_findmetadata(fp, MODINFOMD_ELFHDR)) == NULL)
 		return (EFTYPE);
 
+	riscv_set_boot_hart(fp);
+
 	e = (Elf_Ehdr *)&fmp->md_data;
 
 	efi_time_fini();