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.

Fixes QubesOS/qubes-issues#7332.

patch-Linux-loop-use-direct-io.patch

@@ -0,0 +1,11 @@
+--- 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 16:18:36.667000000 -0500
+@@ -330,7 +330,7 @@
+         else
+           roflag=''
+         fi
+-        do_or_die losetup $roflag "$loopdev" "$file"
++        do_or_die losetup --direct-io=on $roflag "$loopdev" "$file"
+         xenstore_write "$XENBUS_PATH/node" "$loopdev"
+         write_dev "$loopdev"
+         release_lock "block"

patch-xen-hotplug-external-store.patch

@@ -25,7 +25,7 @@ Signed-off-by: Marek Marczykowski <[email protected]>
      FRONTEND_UUID=$(xenstore_read_default \
 @@ -305,6 +312,7 @@
          fi
-         do_or_die losetup $roflag "$loopdev" "$file"
+         do_or_die losetup --direct-io=on $roflag "$loopdev" "$file"
          xenstore_write "$XENBUS_PATH/node" "$loopdev"
 +        echo $loopdev > "$HOTPLUG_STORE-node"
          write_dev "$loopdev"

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-Linux-loop-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-Linux-loop-use-direct-io.patch

xen.spec.in

@@ -201,6 +201,7 @@ 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-Linux-loop-use-direct-io.patch
 
 # GCC8 fixes
 Patch714: patch-tools-kdd-mute-spurious-gcc-warning.patch