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" <address@hidden>
>
> 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 <address@hidden>
> ---
>   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 {