Re: [Xen-devel] [PATCH v3 04/13] xen/pvcalls: implement socket command and handle events

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sat, 12 Aug 2017, Boris Ostrovsky wrote:
> On 07/31/2017 06:57 PM, Stefano Stabellini wrote:
> > Send a PVCALLS_SOCKET command to the backend, use the masked
> > req_prod_pvt as req_id. This way, req_id is guaranteed to be between 0
> > and PVCALLS_NR_REQ_PER_RING. We already have a slot in the rsp array
> > ready for the response, and there cannot be two outstanding responses
> > with the same req_id.
> > 
> > Wait for the response by waiting on the inflight_req waitqueue and
> > check for the req_id field in rsp[req_id]. Use atomic accesses and
> > barriers to read the field. Note that the barriers are simple smp
> > barriers (as opposed to virt barriers) because they are for internal
> > frontend synchronization, not frontend<->backend communication.
> > 
> > Once a response is received, clear the corresponding rsp slot by setting
> > req_id to PVCALLS_INVALID_ID. Note that PVCALLS_INVALID_ID is invalid
> > only from the frontend point of view. It is not part of the PVCalls
> > protocol.
> > 
> > pvcalls_front_event_handler is in charge of copying responses from the
> > ring to the appropriate rsp slot. It is done by copying the body of the
> > response first, then by copying req_id atomically. After the copies,
> > wake up anybody waiting on waitqueue.
> > 
> > pvcallss_lock protects accesses to the ring.
> > 
> > Create a new struct sock_mapping and convert the pointer into an
> > uint64_t and use it as id for the new socket to pass to the backend. The
> > struct will be fully initialized later on connect or bind. In this patch
> > the struct sock_mapping is empty, the fields will be added by the next
> > patch.
> > 
> > sock->sk->sk_send_head is not used for ip sockets: reuse the field to
> > store a pointer to the struct sock_mapping corresponding to the socket.
> > This way, we can easily get the struct sock_mapping from the struct
> > socket.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
> > CC: boris.ostrovsky@xxxxxxxxxx
> > CC: jgross@xxxxxxxx
> > ---
> >   drivers/xen/pvcalls-front.c | 119
> > ++++++++++++++++++++++++++++++++++++++++++++
> >   drivers/xen/pvcalls-front.h |   8 +++
> >   2 files changed, 127 insertions(+)
> >   create mode 100644 drivers/xen/pvcalls-front.h
> > 
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index 2afe36d..7c4a7cb 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -20,6 +20,8 @@
> >   #include <xen/xenbus.h>
> >   #include <xen/interface/io/pvcalls.h>
> >   +#include "pvcalls-front.h"
> > +
> >   #define PVCALLS_INVALID_ID UINT_MAX
> >   #define PVCALLS_RING_ORDER XENBUS_MAX_RING_GRANT_ORDER
> >   #define PVCALLS_NR_REQ_PER_RING __CONST_RING_SIZE(xen_pvcalls,
> > XEN_PAGE_SIZE)
> > @@ -38,11 +40,128 @@ struct pvcalls_bedata {
> >   };
> >   static struct xenbus_device *pvcalls_front_dev;
> >   +struct sock_mapping {
> > +   bool active_socket;
> > +   struct list_head list;
> > +   struct socket *sock;
> > +};
> > +
> > +static inline int get_request(struct pvcalls_bedata *bedata, int *req_id)
> > +{
> > +   *req_id = bedata->ring.req_prod_pvt & (RING_SIZE(&bedata->ring) - 1);
> > +   if (RING_FULL(&bedata->ring) ||
> > +       READ_ONCE(bedata->rsp[*req_id].req_id) != PVCALLS_INVALID_ID)
> > +           return -EAGAIN;
> > +   return 0;
> > +}
> > +
> >   static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
> >   {
> > +   struct xenbus_device *dev = dev_id;
> > +   struct pvcalls_bedata *bedata;
> > +   struct xen_pvcalls_response *rsp;
> > +   uint8_t *src, *dst;
> > +   int req_id = 0, more = 0, done = 0;
> > +
> > +   if (dev == NULL)
> > +           return IRQ_HANDLED;
> > +
> > +   bedata = dev_get_drvdata(&dev->dev);
> > +   if (bedata == NULL)
> > +           return IRQ_HANDLED;
> > +
> > +again:
> > +   while (RING_HAS_UNCONSUMED_RESPONSES(&bedata->ring)) {
> > +           rsp = RING_GET_RESPONSE(&bedata->ring, bedata->ring.rsp_cons);
> > +
> > +           req_id = rsp->req_id;
> > +           dst = (uint8_t *)&bedata->rsp[req_id] + sizeof(rsp->req_id);
> > +           src = (uint8_t *)rsp + sizeof(rsp->req_id);
> > +           memcpy(dst, src, sizeof(*rsp) - sizeof(rsp->req_id));
> > +           /*
> > +            * First copy the rest of the data, then req_id. It is
> > +            * paired with the barrier when accessing bedata->rsp.
> > +            */
> > +           smp_wmb();
> > +           WRITE_ONCE(bedata->rsp[req_id].req_id, rsp->req_id);
> > +
> > +           done = 1;
> > +           bedata->ring.rsp_cons++;
> > +   }
> > +
> > +   RING_FINAL_CHECK_FOR_RESPONSES(&bedata->ring, more);
> > +   if (more)
> > +           goto again;
> > +   if (done)
> > +           wake_up(&bedata->inflight_req);
> >     return IRQ_HANDLED;
> >   }
> >   +int pvcalls_front_socket(struct socket *sock)
> > +{
> > +   struct pvcalls_bedata *bedata;
> > +   struct sock_mapping *map = NULL;
> > +   struct xen_pvcalls_request *req;
> > +   int notify, req_id, ret;
> > +
> > +   if (!pvcalls_front_dev)
> > +           return -EACCES;
> > +   /*
> > +    * PVCalls only supports domain AF_INET,
> > +    * type SOCK_STREAM and protocol 0 sockets for now.
> > +    *
> > +    * Check socket type here, AF_INET and protocol checks are done
> > +    * by the caller.
> > +    */
> > +   if (sock->type != SOCK_STREAM)
> > +       return -ENOTSUPP;
> > +
> > +   bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> > +
> > +   map = kzalloc(sizeof(*map), GFP_KERNEL);
> > +   if (map == NULL)
> > +           return -ENOMEM;
> > +   /*
> > +    * sock->sk->sk_send_head is not used for ip sockets: reuse the
> > +    * field to store a pointer to the struct sock_mapping
> > +    * corresponding to the socket. This way, we can easily get the
> > +    * struct sock_mapping from the struct socket.
> 
> 
> Is this a safe assumption? Is it conceivable that at some point in the future
> sk_send_head might become used?

I don't think there is any risk of sk_send_head being used anytime soon:
among all the  protocols supported under net/, sk_send_head is only used
by dccp.


> > +    */
> > +   WRITE_ONCE(sock->sk->sk_send_head, (void *)map);
> > +
> > +   spin_lock(&bedata->pvcallss_lock);
> > +   list_add_tail(&map->list, &bedata->socket_mappings);
> > +
> > +   ret = get_request(bedata, &req_id);
> > +   if (ret < 0) {
> > +           spin_unlock(&bedata->pvcallss_lock);
> > +           return ret;
> What is happening to the mapping you've linked above? Should it be unlinked
> and freed? (or defer its creation until we've got a request slot?)

Yes, you are right.


> > +   }
> > +   req = RING_GET_REQUEST(&bedata->ring, req_id);
> > +   req->req_id = req_id;
> > +   req->cmd = PVCALLS_SOCKET;
> > +   req->u.socket.id = (uint64_t) map;
> > +   req->u.socket.domain = AF_INET;
> > +   req->u.socket.type = SOCK_STREAM;
> > +   req->u.socket.protocol = 0;
> 
> IPPROTO_IP?

Yes, thanks

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