efi loader disk order

[Nick Owens <mischief@offblast.org>]

on my desktop i have multiple disks. one contains a full 9front install
in uefi mode with an esp and 9fat. in my primary disk, i added
bootx64.efi, 9pc64 and a plan9.ini and configured grub to chainload it,
so i could use those to tcpboot to my fileserver.

i was very surprised when bootx64.efi picked the plan9.ini from the
other disk with the full 9front installation! 9boot(8) is somewhat
unspecific on the behavior, but the current code seems to walk all disks
in reverse order looking for plan9.ini in them. i understand the intent
of this ordering was to select the 9fat first (which would usually be
subsequent to the ESP), but i do not understand why 9front doesn't just
forgo a 9fat if an ESP is present.

in any case, here's a patch to load plan9.ini and the kernel from the
same partition that bootx64.efi was loaded from, and if that fails, fall
back to the old behavior of picking whatever.

diff --git a/sys/src/boot/efi/efi.h b/sys/src/boot/efi/efi.h
--- a/sys/src/boot/efi/efi.h
+++ b/sys/src/boot/efi/efi.h
@@ -49,6 +49,22 @@ typedef struct {
 } EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL;
 
 typedef struct {
+	UINT32		Revision;
+	EFI_HANDLE	ParentHandle;
+	void		*SystemTable;
+	EFI_HANDLE	DeviceHandle;
+	void		*FilePath;
+	void		*Reserved;
+	UINT32		LoadOptionsSize;
+	void		*LoadOptions;
+	void		*ImageBase;
+	UINT64		ImageSize;
+	UINT32		ImageCodeType;
+	UINT32		ImageDataType;
+	void		*Unload;
+} EFI_LOADED_IMAGE_PROTOCOL;
+
+typedef struct {
 	UINT32		RedMask;
 	UINT32		GreenMask;
 	UINT32		BlueMask;
diff --git a/sys/src/boot/efi/fs.c b/sys/src/boot/efi/fs.c
--- a/sys/src/boot/efi/fs.c
+++ b/sys/src/boot/efi/fs.c
@@ -32,6 +32,12 @@ EFI_GUID EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
 	0xc9, 0x69, 0x72, 0x3b,
 };
 
+static EFI_GUID EFI_LOADED_IMAGE_PROTOCOL_GUID = {
+	0x5b1b31a1, 0x9562, 0x11d2,
+	0x8e, 0x3f, 0x00, 0xa0,
+	0xc9, 0x69, 0x72, 0x3b,
+};
+
 static
 EFI_FILE_PROTOCOL *fsroot;
 
@@ -87,12 +93,37 @@ int
 fsinit(void **pf)
 {
 	EFI_SIMPLE_FILE_SYSTEM_PROTOCOL *fs;
+	EFI_LOADED_IMAGE_PROTOCOL *image;
 	EFI_FILE_PROTOCOL *root;
 	EFI_HANDLE *Handles;
 	void *f;
 	UINTN Count;
 	int i;
 
+	image = nil;
+
+	/* locate kernel and plan9.ini by deriving a fs protocol
+	 * from the device the loader was read from.
+	 * if that fails, fall back to old method.
+	 */
+	if(eficall(ST->BootServices->HandleProtocol, IH,
+		&EFI_LOADED_IMAGE_PROTOCOL_GUID, &image) == 0 &&
+		eficall(ST->BootServices->HandleProtocol, image->DeviceHandle,
+		&EFI_SIMPLE_FILE_SYSTEM_PROTOCOL_GUID, &fs) == 0 &&
+		eficall(fs->OpenVolume, fs, &root) == 0){
+
+		fsroot = root;
+		f = fsopen("/plan9.ini");
+		if(f != nil){
+			if(pf != nil)
+				*pf = f;
+			else
+				fsclose(f);
+
+			goto gotit;
+		}
+	}
+
 	Count = 0;
 	Handles = nil;
 	if(eficall(ST->BootServices->LocateHandleBuffer,
@@ -126,6 +157,7 @@ fsinit(void **pf)
 	if(fsroot == nil)
 		return -1;
 
+gotit:
 	read = fsread;
 	close = fsclose;
 	open = fsopen;