[PATCH] efi: fix semihosting EFI payload booting

[Andre Przywara]

At the moment any naive attempt to boot an EFI payload that has just
been loaded via "hostfs" (sandbox or semihosting) is met by a rather
confusing error message:
===========
VExpress64# load hostfs - $kernel_addr_r Image
52752896 bytes read in 8 ms (6.1 GiB/s)
VExpress64# bootefi $kernel_addr_r
No UEFI binary known at 0x80080000
===========
Actually explicitly providing the filesize:
VExpress64# bootefi $kernel_addr_r:$filesize
works around that problem, but the issue lies deeper: the call to
efi_set_bootdev() (as done by the generic load code) bails out at some
point, leaving the image_addr and image_size variables unset, which
triggers this message. The problem seems to be that "-" is not
understood by the code creating an UEFI device path. We could try to fix
just that, but actually semihosting seems to have some explicit support
in UEFI (at least it does in EDK II): there is a separate GUID for it,
and hostfs is significantly different in some aspects to justify special
handling.

Check for the device name being "hostfs" and create a specific UEFI device
path for semihosting in this case. This uses the GUID used by EDK II for
almost 15 years.
This fixes the above load/bootefi sequence without requiring an explicit
file size argument.

Signed-off-by: Andre Przywara <andre.przywara at arm.com>
---
 lib/efi_loader/efi_device_path.c | 34 ++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/lib/efi_loader/efi_device_path.c b/lib/efi_loader/efi_device_path.c
index e2e98a39be1..b6d2074dd70 100644
--- a/lib/efi_loader/efi_device_path.c
+++ b/lib/efi_loader/efi_device_path.c
@@ -1079,6 +1079,35 @@ struct efi_device_path *efi_dp_from_uart(void)
 	return buf;
 }
 
+#define SEMIHOSTING_GUID \
+	EFI_GUID(0xc5b9c74a, 0x6d72, 0x4719, \
+		 0x99, 0xab, 0xc5, 0x9f, 0x19, 0x90, 0x91, 0xeb)
+
+struct efi_device_path *efi_dp_from_semihosting(void)
+{
+	const struct efi_device_path_vendor smh_vendor = {
+		.dp = {
+			.type     = DEVICE_PATH_TYPE_HARDWARE_DEVICE,
+			.sub_type = DEVICE_PATH_SUB_TYPE_VENDOR,
+			.length   = sizeof(smh_vendor),
+		},
+		.guid = SEMIHOSTING_GUID,
+	};
+	void *buf, *pos;
+	size_t dpsize = sizeof(smh_vendor) + sizeof(END);
+
+	buf = efi_alloc(dpsize);
+	if (!buf)
+		return NULL;
+	pos = buf;
+	memcpy(pos, &smh_vendor, sizeof(smh_vendor));
+	pos += sizeof(smh_vendor);
+
+	memcpy(pos, &END, sizeof(END));
+
+	return buf;
+}
+
 #ifdef CONFIG_NETDEVICES
 struct efi_device_path *efi_dp_from_eth(void)
 {
@@ -1210,6 +1239,11 @@ efi_status_t efi_dp_from_name(const char *dev, const char *devnr,
 			*device = efi_dp_from_mem(EFI_RESERVED_MEMORY_TYPE,
 						  (uintptr_t)image_addr,
 						  image_size);
+	} else if (!strcmp(dev, "hostfs")) {
+		efi_get_image_parameters(&image_addr, &image_size);
+
+		if (device)
+			*device = efi_dp_from_semihosting();
 	} else {
 		part = blk_get_device_part_str(dev, devnr, &desc, &fs_partition,
 					       1);
-- 
2.25.1