Use direct I/O for loop devices

This is a huge performance improvement for two reasons:

1. It uses the filesystem’s asynchronous I/O support, rather than using
   synchronous I/O.
2. It bypasses the page cache, removing a redundant layer of caching and
   associated overhead.

I also took the opportunity to rip out some cruft related to old losetup
versions, which Qubes OS doesn't need to support anymore.

Fixes QubesOS/qubes-issues#7332.

patch-use-direct-io.patch

@@ -0,0 +1,55 @@
+--- xen-4.14.4/tools/hotplug/Linux/block	2022-01-31 04:44:39.000000000 -0500
++++ xen-4.14.4/tools/hotplug/Linux/block	2022-03-11 15:17:37.410000000 -0500
+@@ -16,27 +16,6 @@
+   echo -n $dev
+ }
+
+-find_free_loopback_helper() {
+-  local next_devnum=0
+-  local busy_devnum
+-  while read busy_devnum; do
+-    if [ "$next_devnum" != "$busy_devnum" ]; then
+-      break
+-    fi
+-    let next_devnum=$next_devnum+1
+-  done
+-  echo "/dev/loop${next_devnum}"
+-}
+-
+-# Not all distros have "losetup -f"
+-find_free_loopback_dev() {
+-  local loopdev
+-  loopdev=$(losetup -a | sed -e 's+^/dev/loop++' -e 's/:.*//' | find_free_loopback_helper)
+-  if [ -n "$loopdev" ] && [ -b "$loopdev" ]; then
+-    echo "$loopdev"
+-  fi
+-}
+-
+ ##
+ # check_sharing devtype device mode [inode]
+ #
+@@ -317,20 +296,16 @@
+           check_file_sharing "$file" "$dev" "$mode" "$inode"
+         fi
+
+-        loopdev=$(losetup -f 2>/dev/null || find_free_loopback_dev)
++        loopdev=$(losetup -f 2>/dev/null)
+         if [ "$loopdev" = '' ]
+         then
+           release_lock "block"
+           fatal 'Failed to find an unused loop device'
+         fi
+
+-        if LANG=C losetup -h 2>&1 | grep read-only >/dev/null
+-        then
+-          roflag="-$mode"; roflag="${roflag#-w}"; roflag="${roflag#-!}"
+-        else
+-          roflag=''
+-        fi
+-        do_or_die losetup $roflag "$loopdev" "$file"
++        roflag="-$mode"
++        roflag="${roflag#-w}"
++        do_or_die losetup --direct-io=on ${roflag#-!} "$loopdev" "$file"
+         xenstore_write "$XENBUS_PATH/node" "$loopdev"
+         write_dev "$loopdev"
+         release_lock "block"

series-debian-vm.conf

@@ -18,3 +18,4 @@ patch-Define-build-dates-time-based-on-SOURCE_DATE_EPOCH.patch
 patch-docs-rename-DATE-to-PANDOC_REL_DATE-and-allow-to-spe.patch
 patch-docs-xen-headers-use-alphabetical-sorting-for-incont.patch
 patch-Strip-build-path-directories-in-tools-xen-and-xen-ar.patch
+patch-use-direct-io.patch

series-vm.conf

@@ -8,3 +8,4 @@ patch-Define-build-dates-time-based-on-SOURCE_DATE_EPOCH.patch
 patch-docs-rename-DATE-to-PANDOC_REL_DATE-and-allow-to-spe.patch
 patch-docs-xen-headers-use-alphabetical-sorting-for-incont.patch
 patch-Strip-build-path-directories-in-tools-xen-and-xen-ar.patch
+patch-use-direct-io.patch

xen.spec.in

@@ -167,6 +167,7 @@ Patch636: patch-autoconf-fix-handling-absolute-PYTHON-path.patch
 Patch638: patch-libxl-do-not-require-filling-backend_domid-to-remove.patch
 Patch639: patch-libxl-add-pcidevs-to-stubdomain-earlier.patch
 Patch640: patch-xen-efi-align.patch
+Patch641: patch-use-direct-io.patch
 
 # GCC8 fixes
 Patch714: patch-tools-kdd-mute-spurious-gcc-warning.patch