From 64fa56e0520215e3909e442f09d8073c1870648a Mon Sep 17 00:00:00 2001
From: Thomas Huth <thuth@redhat.com>
Date: Fri, 8 Jul 2022 20:49:01 +0200
Subject: [PATCH 07/17] pc-bios/s390-ccw/bootmap: Improve the guessing logic in
 zipl_load_vblk()

RH-Author: Thomas Huth <thuth@redhat.com>
RH-MergeRequest: 106: pc-bios/s390-ccw: Fix boot from disks with 4k sectors that do not have the typical DASD geometry
RH-Commit: [2/10] ca8f5e847617cf4ac2fd6c38edb2982f32fa3eba (thuth/qemu-kvm-cs9)
RH-Bugzilla: 2098077
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>

Bugzilla: http://bugzilla.redhat.com/2098077

commit 422865f6672ee1482b98d18321b55c1ecfb06c82
Author: Thomas Huth <thuth@redhat.com>
Date:   Mon Jul 4 13:18:54 2022 +0200

    pc-bios/s390-ccw/bootmap: Improve the guessing logic in zipl_load_vblk()

    The logic of trying an final ISO or ECKD boot on virtio-block devices is
    very weird: Since the geometry hardly ever matches in virtio_disk_is_scsi(),
    virtio_blk_setup_device() always sets a "guessed" disk geometry via
    virtio_assume_scsi() (which is certainly also wrong in a lot of cases).

    zipl_load_vblk() then sees that there's been a "virtio_guessed_disk_nature"
    and tries to fix up the geometry again via virtio_assume_iso9660() before
    always trying to do ipl_iso_el_torito(). That's a very brain-twisting
    way of attempting to boot from ISO images, which won't work anymore after
    the following patches that will clean up the virtio_assume_scsi() mess
    (and thus get rid of the "virtio_guessed_disk_nature" here).

    Let's try a better approach instead: ISO files always have a magic
    string "CD001" at offset 0x8001 (see e.g. the ECMA-119 specification)
    which we can use to decide whether we should try to boot in ISO 9660
    mode (which we should also try if we see a sector size of 2048).

    And if we were not able to boot in ISO mode here, the final boot attempt
    before panicking is to boot in ECKD mode. Since this is our last boot
    attempt anyway, simply always assume the ECKD geometry here (if the sector
    size was not 4096 yet), so that we also do not depend on the guessed disk
    geometry from virtio_blk_setup_device() here anymore.

    Message-Id: <20220704111903.62400-4-thuth@redhat.com>
    Signed-off-by: Thomas Huth <thuth@redhat.com>

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 pc-bios/s390-ccw/bootmap.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/pc-bios/s390-ccw/bootmap.c b/pc-bios/s390-ccw/bootmap.c
index 56411ab3b6..994e59c0b0 100644
--- a/pc-bios/s390-ccw/bootmap.c
+++ b/pc-bios/s390-ccw/bootmap.c
@@ -780,18 +780,37 @@ static void ipl_iso_el_torito(void)
     }
 }
 
+/**
+ * Detect whether we're trying to boot from an .ISO image.
+ * These always have a signature string "CD001" at offset 0x8001.
+ */
+static bool has_iso_signature(void)
+{
+    int blksize = virtio_get_block_size();
+
+    if (!blksize || virtio_read(0x8000 / blksize, sec)) {
+        return false;
+    }
+
+    return !memcmp("CD001", &sec[1], 5);
+}
+
 /***********************************************************************
  * Bus specific IPL sequences
  */
 
 static void zipl_load_vblk(void)
 {
-    if (virtio_guessed_disk_nature()) {
-        virtio_assume_iso9660();
+    int blksize = virtio_get_block_size();
+
+    if (blksize == VIRTIO_ISO_BLOCK_SIZE || has_iso_signature()) {
+        if (blksize != VIRTIO_ISO_BLOCK_SIZE) {
+            virtio_assume_iso9660();
+        }
+        ipl_iso_el_torito();
     }
-    ipl_iso_el_torito();
 
-    if (virtio_guessed_disk_nature()) {
+    if (blksize != VIRTIO_DASD_DEFAULT_BLOCK_SIZE) {
         sclp_print("Using guessed DASD geometry.\n");
         virtio_assume_eckd();
     }
-- 
2.31.1