[PATCH-for-5.2 v2 4/4] configure: mark vhost-user Linux-only

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

From: Stefan Hajnoczi <stefanha@redhat.com>

The vhost-user protocol uses the Linux eventfd feature and is typically
connected to Linux kvm.ko ioeventfd and irqfd file descriptors. The
protocol specification in docs/interop/vhost-user.rst does not describe
how platforms without eventfd support work.

The QEMU vhost-user devices compile on other POSIX host operating
systems because eventfd usage is abstracted in QEMU. The libvhost-user
programs in contrib/ do not compile but we failed to notice since they
are not built by default.

Make it clear that vhost-user is only supported on Linux for the time
being. If someone wishes to support it on other platforms then the
details can be added to vhost-user.rst and CI jobs can test the feature
to prevent bitrot.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201110171121.1265142-4-stefanha@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 configure | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/configure b/configure
index 516f28a0888..3fbc2a0c68c 100755
--- a/configure
+++ b/configure
@@ -328,7 +328,7 @@ vhost_net=""
 vhost_crypto=""
 vhost_scsi=""
 vhost_vsock=""
-vhost_user=""
+vhost_user="no"
 vhost_user_blk_server="auto"
 vhost_user_fs=""
 kvm="auto"
@@ -718,7 +718,6 @@ fi
 case $targetos in
 MINGW32*)
   mingw32="yes"
-  vhost_user="no"
   audio_possible_drivers="dsound sdl"
   if check_include dsound.h; then
     audio_drv_list="dsound"
@@ -797,6 +796,7 @@ Linux)
   audio_possible_drivers="oss alsa sdl pa"
   linux="yes"
   linux_user="yes"
+  vhost_user="yes"
 ;;
 esac
 
@@ -2341,9 +2341,8 @@ fi
 # vhost interdependencies and host support
 
 # vhost backends
-test "$vhost_user" = "" && vhost_user=yes
-if test "$vhost_user" = "yes" && test "$mingw32" = "yes"; then
-  error_exit "vhost-user isn't available on win32"
+if test "$vhost_user" = "yes" && test "$linux" != "yes"; then
+  error_exit "vhost-user is only available on Linux"
 fi
 test "$vhost_vdpa" = "" && vhost_vdpa=$linux
 if test "$vhost_vdpa" = "yes" && test "$linux" != "yes"; then
-- 
2.26.2