Re: [Qemu-devel] [PATCH v3 15/29] vhost+postcopy: Send address back to qemu

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 27, 2018 at 07:54:18PM +0000, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) wrote:
> > On Fri, Feb 16, 2018 at 01:16:11PM +0000, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > 
> > > We need a better way, but at the moment we need the address of the
> > > mappings sent back to qemu so it can interpret the messages on the
> > > userfaultfd it reads.
> > > 
> > > This is done as a 3 stage set:
> > >    QEMU -> client
> > >       set_mem_table
> > > 
> > >    mmap stuff, get addresses
> > > 
> > >    client -> qemu
> > >        here are the addresses
> > > 
> > >    qemu -> client
> > >        OK - now you can use them
> > > 
> > > That ensures that qemu has registered the new addresses in it's
> > > userfault code before the client starts accessing them.
> > > 
> > > Note: We don't ask for the default 'ack' reply since we've got our own.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > ---
> > >  contrib/libvhost-user/libvhost-user.c | 24 ++++++++++++-
> > >  docs/interop/vhost-user.txt           |  9 +++++
> > >  hw/virtio/trace-events                |  1 +
> > >  hw/virtio/vhost-user.c                | 67 +++++++++++++++++++++++++++++++++--
> > >  4 files changed, 98 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/contrib/libvhost-user/libvhost-user.c b/contrib/libvhost-user/libvhost-user.c
> > > index a18bc74a7c..e02e5d6f46 100644
> > > --- a/contrib/libvhost-user/libvhost-user.c
> > > +++ b/contrib/libvhost-user/libvhost-user.c
> > > @@ -491,10 +491,32 @@ vu_set_mem_table_exec_postcopy(VuDev *dev, VhostUserMsg *vmsg)
> > >                     dev_region->mmap_addr);
> > >          }
> > >  
> > > +        /* Return the address to QEMU so that it can translate the ufd
> > > +         * fault addresses back.
> > > +         */
> > > +        msg_region->userspace_addr = (uintptr_t)(mmap_addr +
> > > +                                                 dev_region->mmap_offset);
> > >          close(vmsg->fds[i]);
> > >      }
> > >  
> > > -    /* TODO: Get address back to QEMU */
> > > +    /* Send the message back to qemu with the addresses filled in */
> > > +    vmsg->fd_num = 0;
> > > +    if (!vu_message_write(dev, dev->sock, vmsg)) {
> > > +        vu_panic(dev, "failed to respond to set-mem-table for postcopy");
> > > +        return false;
> > > +    }
> > > +
> > > +    /* Wait for QEMU to confirm that it's registered the handler for the
> > > +     * faults.
> > > +     */
> > > +    if (!vu_message_read(dev, dev->sock, vmsg) ||
> > > +        vmsg->size != sizeof(vmsg->payload.u64) ||
> > > +        vmsg->payload.u64 != 0) {
> > > +        vu_panic(dev, "failed to receive valid ack for postcopy set-mem-table");
> > > +        return false;
> > > +    }
> > > +
> > > +    /* OK, now we can go and register the memory and generate faults */
> > >      for (i = 0; i < dev->nregions; i++) {
> > >          VuDevRegion *dev_region = &dev->regions[i];
> > >  #ifdef UFFDIO_REGISTER
> > > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> > > index bdec9ec0e8..5bbcab2cc4 100644
> > > --- a/docs/interop/vhost-user.txt
> > > +++ b/docs/interop/vhost-user.txt
> > > @@ -454,12 +454,21 @@ Master message types
> > >        Id: 5
> > >        Equivalent ioctl: VHOST_SET_MEM_TABLE
> > >        Master payload: memory regions description
> > > +      Slave payload: (postcopy only) memory regions description
> > >  
> > >        Sets the memory map regions on the slave so it can translate the vring
> > >        addresses. In the ancillary data there is an array of file descriptors
> > >        for each memory mapped region. The size and ordering of the fds matches
> > >        the number and ordering of memory regions.
> > >  
> > > +      When postcopy-listening has been received,
> > 
> > Which message is this?
> 
> VHOST_USER_POSTCOPY_LISTEN
> 
> Do you want me just to change that to, 'When VHOST_USER_POSTCOPY_LISTEN
> has been received' ?

I think it's better this way, yes.

> > > SET_MEM_TABLE replies with
> > > +      the bases of the memory mapped regions to the master.  It must have mmap'd
> > > +      the regions but not yet accessed them and should not yet generate a userfault
> > > +      event. Note NEED_REPLY_MASK is not set in this case.
> > > +      QEMU will then reply back to the list of mappings with an empty
> > > +      VHOST_USER_SET_MEM_TABLE as an acknolwedgment; only upon reception of this
> > > +      message may the guest start accessing the memory and generating faults.
> > > +
> > >   * VHOST_USER_SET_LOG_BASE
> > >  
> > >        Id: 6
> > 
> > As you say yourself, this is probably the best we can do for now,
> > but it's not ideal. So I think it's a good idea to isolate this
> > behind a separate protocol feature bit. For now it will be required
> > for postcopy, when it's fixed in kernel we can drop it
> > cleanly.
> > 
> 
> While we've talked about ways of avoiding the exact addresses being
> known by the slave, I'm not sure we've talked about a way of removing
> this handshake; although it's doable if we move more of the work to the QEMU
> side.
> 
> Dave

Some kernel changes might thinkably remove the need for use of the
address with userfaultfd, too.

> > > diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> > > index 06ec03d6e7..05d18ada77 100644
> > > --- a/hw/virtio/trace-events
> > > +++ b/hw/virtio/trace-events
> > > @@ -8,6 +8,7 @@ vhost_section(const char *name, int r) "%s:%d"
> > >  
> > >  # hw/virtio/vhost-user.c
> > >  vhost_user_postcopy_listen(void) ""
> > > +vhost_user_set_mem_table_postcopy(uint64_t client_addr, uint64_t qhva, int reply_i, int region_i) "client:0x%"PRIx64" for hva: 0x%"PRIx64" reply %d region %d"
> > >  
> > >  # hw/virtio/virtio.c
> > >  virtqueue_alloc_element(void *elem, size_t sz, unsigned in_num, unsigned out_num) "elem %p size %zd in_num %u out_num %u"
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index 64f4b3b3f9..a060442cb9 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -159,6 +159,7 @@ struct vhost_user {
> > >      int slave_fd;
> > >      NotifierWithReturn postcopy_notifier;
> > >      struct PostCopyFD  postcopy_fd;
> > > +    uint64_t           postcopy_client_bases[VHOST_MEMORY_MAX_NREGIONS];
> > >      /* True once we've entered postcopy_listen */
> > >      bool               postcopy_listen;
> > >  };
> > > @@ -328,12 +329,15 @@ static int vhost_user_set_log_base(struct vhost_dev *dev, uint64_t base,
> > >  static int vhost_user_set_mem_table_postcopy(struct vhost_dev *dev,
> > >                                               struct vhost_memory *mem)
> > >  {
> > > +    struct vhost_user *u = dev->opaque;
> > >      int fds[VHOST_MEMORY_MAX_NREGIONS];
> > >      int i, fd;
> > >      size_t fd_num = 0;
> > >      bool reply_supported = virtio_has_feature(dev->protocol_features,
> > >                                                VHOST_USER_PROTOCOL_F_REPLY_ACK);
> > > -    /* TODO: Add actual postcopy differences */
> > > +    VhostUserMsg msg_reply;
> > > +    int region_i, msg_i;
> > > +
> > >      VhostUserMsg msg = {
> > >          .hdr.request = VHOST_USER_SET_MEM_TABLE,
> > >          .hdr.flags = VHOST_USER_VERSION,
> > > @@ -380,6 +384,64 @@ static int vhost_user_set_mem_table_postcopy(struct vhost_dev *dev,
> > >          return -1;
> > >      }
> > >  
> > > +    if (vhost_user_read(dev, &msg_reply) < 0) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (msg_reply.hdr.request != VHOST_USER_SET_MEM_TABLE) {
> > > +        error_report("%s: Received unexpected msg type."
> > > +                     "Expected %d received %d", __func__,
> > > +                     VHOST_USER_SET_MEM_TABLE, msg_reply.hdr.request);
> > > +        return -1;
> > > +    }
> > > +    /* We're using the same structure, just reusing one of the
> > > +     * fields, so it should be the same size.
> > > +     */
> > > +    if (msg_reply.hdr.size != msg.hdr.size) {
> > > +        error_report("%s: Unexpected size for postcopy reply "
> > > +                     "%d vs %d", __func__, msg_reply.hdr.size, msg.hdr.size);
> > > +        return -1;
> > > +    }
> > > +
> > > +    memset(u->postcopy_client_bases, 0,
> > > +           sizeof(uint64_t) * VHOST_MEMORY_MAX_NREGIONS);
> > > +
> > > +    /* They're in the same order as the regions that were sent
> > > +     * but some of the regions were skipped (above) if they
> > > +     * didn't have fd's
> > > +    */
> > > +    for (msg_i = 0, region_i = 0;
> > > +         region_i < dev->mem->nregions;
> > > +        region_i++) {
> > > +        if (msg_i < fd_num &&
> > > +            msg_reply.payload.memory.regions[msg_i].guest_phys_addr ==
> > > +            dev->mem->regions[region_i].guest_phys_addr) {
> > > +            u->postcopy_client_bases[region_i] =
> > > +                msg_reply.payload.memory.regions[msg_i].userspace_addr;
> > > +            trace_vhost_user_set_mem_table_postcopy(
> > > +                msg_reply.payload.memory.regions[msg_i].userspace_addr,
> > > +                msg.payload.memory.regions[msg_i].userspace_addr,
> > > +                msg_i, region_i);
> > > +            msg_i++;
> > > +        }
> > > +    }
> > > +    if (msg_i != fd_num) {
> > > +        error_report("%s: postcopy reply not fully consumed "
> > > +                     "%d vs %zd",
> > > +                     __func__, msg_i, fd_num);
> > > +        return -1;
> > > +    }
> > > +    /* Now we've registered this with the postcopy code, we ack to the client,
> > > +     * because now we're in the position to be able to deal with any faults
> > > +     * it generates.
> > > +     */
> > > +    /* TODO: Use this for failure cases as well with a bad value */
> > > +    msg.hdr.size = sizeof(msg.payload.u64);
> > > +    msg.payload.u64 = 0; /* OK */
> > > +    if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
> > > +        return -1;
> > > +    }
> > > +
> > >      if (reply_supported) {
> > >          return process_message_reply(dev, &msg);
> > >      }
> > > @@ -396,7 +458,8 @@ static int vhost_user_set_mem_table(struct vhost_dev *dev,
> > >      size_t fd_num = 0;
> > >      bool do_postcopy = u->postcopy_listen && u->postcopy_fd.handler;
> > >      bool reply_supported = virtio_has_feature(dev->protocol_features,
> > > -                                              VHOST_USER_PROTOCOL_F_REPLY_ACK);
> > > +                                          VHOST_USER_PROTOCOL_F_REPLY_ACK) &&
> > > +                                          !do_postcopy;
> > >  
> > >      if (do_postcopy) {
> > >          /* Postcopy has enough differences that it's best done in it's own
> > > -- 
> > > 2.14.3
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK