Re: [Qemu-devel] [RFC 10/29] vhub: Open userfaultfd

[Maxime Coquelin]

On 06/28/2017 09:00 PM, Dr. David Alan Gilbert (git) wrote:

From: "Dr. David Alan Gilbert" <dgilb...@redhat.com>

Open a userfaultfd (on a postcopy_advise) and send it back in
the reply to the qemu for it to monitor.

Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
---
  contrib/libvhost-user/libvhost-user.c | 24 +++++++++++++++++++++---
  contrib/libvhost-user/libvhost-user.h |  3 +++
  2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/contrib/libvhost-user/libvhost-user.c 
b/contrib/libvhost-user/libvhost-user.c
index e3a32755cf..62e97f6b84 100644
--- a/contrib/libvhost-user/libvhost-user.c
+++ b/contrib/libvhost-user/libvhost-user.c
@@ -15,6 +15,7 @@

#include <qemu/osdep.h>

  #include <sys/eventfd.h>
+#include <sys/syscall.h>
  #include <linux/vhost.h>

#include "qemu/atomic.h"

@@ -774,11 +775,28 @@ vu_set_vring_enable_exec(VuDev *dev, VhostUserMsg *vmsg)
  static bool
  vu_set_postcopy_advise(VuDev *dev, VhostUserMsg *vmsg)
  {
-    /* TODO: Open ufd, pass it back in the request
-    /* TODO: Add addresses
-     */
+    struct uffdio_api api_struct;
+
+    dev->postcopy_ufd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+    /* TODO: Add addresses */
      vmsg->payload.u64 = 0xcafe;
      vmsg->size = sizeof(vmsg->payload.u64);
+
+    if (dev->postcopy_ufd == -1) {
+        vu_panic(dev, "Userfaultfd not available: %s", strerror(errno));
+        return false;

I think we may want to reply something even in case of error.

Indeed, if something goes wrong on backend side, Qemu will remain
blocked waiting for the reply.

+    }
+    api_struct.api = UFFD_API;
+    api_struct.features = 0;
+    if (ioctl(dev->postcopy_ufd, UFFDIO_API, &api_struct)) {
+        vu_panic(dev, "Failed UFFDIO_API: %s", strerror(errno));
+        close(dev->postcopy_ufd);
+        return false;

Ditto

+    }
+    /* TODO: Stash feature flags somewhere */
+    /* Return a ufd to the QEMU */
+    vmsg->fd_num = 1;
+    vmsg->fds[0] = dev->postcopy_ufd;
      return true; /* = send a reply */
  }

diff --git a/contrib/libvhost-user/libvhost-user.h b/contrib/libvhost-user/libvhost-user.h

index 8bb35582ea..3e65a962da 100644
--- a/contrib/libvhost-user/libvhost-user.h
+++ b/contrib/libvhost-user/libvhost-user.h
@@ -231,6 +231,9 @@ struct VuDev {
       * re-initialize */
      vu_panic_cb panic;
      const VuDevIface *iface;
+
+    /* Postcopy data */
+    int postcopy_ufd;
  };

typedef struct VuVirtqElement {