From 3a1f80e2d770426ab1f121d9925244314c10d897 Mon Sep 17 00:00:00 2001
From: Toomas Soome <tsoome@FreeBSD.org>
Date: Tue, 30 Apr 2019 17:45:22 +0000
Subject: [PATCH] zfsboot: to detect disk size, use GPT information first

If we do have GPT on disk, read the disk size from it and do not
call int13.

Since int13 does report bogus informatiopn too often, rather trust the
partition table. We are using the same strategy with loader.

MFC after:	1 month
---
 stand/i386/zfsboot/zfsboot.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/stand/i386/zfsboot/zfsboot.c b/stand/i386/zfsboot/zfsboot.c
index fe3c5d1e6b07..076f96da05f8 100644
--- a/stand/i386/zfsboot/zfsboot.c
+++ b/stand/i386/zfsboot/zfsboot.c
@@ -459,6 +459,33 @@ copy_dsk(struct zfsdsk *zdsk)
     return (newdsk);
 }
 
+/*
+ * Get disk size from GPT.
+ */
+static uint64_t
+drvsize_gpt(struct dsk *dskp)
+{
+#ifdef GPT
+	struct gpt_hdr hdr;
+	char *sec;
+
+	sec = dmadat->secbuf;
+	if (drvread(dskp, sec, 1, 1))
+		return (0);
+
+	memcpy(&hdr, sec, sizeof(hdr));
+	if (memcmp(hdr.hdr_sig, GPT_HDR_SIG, sizeof(hdr.hdr_sig)) != 0 ||
+	    hdr.hdr_lba_self != 1 || hdr.hdr_revision < 0x00010000 ||
+	    hdr.hdr_entsz < sizeof(struct gpt_ent) ||
+	    DEV_BSIZE % hdr.hdr_entsz != 0) {
+		return (0);
+	}
+	return (hdr.hdr_lba_alt + 1);
+#else
+	return (0);
+#endif
+}
+
 /*
  * Get disk size from eax=0x800 and 0x4800. We need to probe both
  * because 0x4800 may not be available and we would like to get more
@@ -475,6 +502,11 @@ drvsize_ext(struct zfsdsk *zdsk)
 
 	dskp = &zdsk->dsk;
 
+	/* Try to read disk size from GPT */
+	size = drvsize_gpt(dskp);
+	if (size != 0)
+		return (size);
+
 	v86.ctl = V86_FLAGS;
 	v86.addr = 0x13;
 	v86.eax = 0x800;