Re: [Xen-devel] [PATCH v1 03/13] xen/pvcalls: implement socket command and handle events

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 24 Jul 2017, Juergen Gross wrote:
> On 22/07/17 02:11, 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 to
> > read the field. 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.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
> > CC: boris.ostrovsky@xxxxxxxxxx
> > CC: jgross@xxxxxxxx
> > ---
> >  drivers/xen/pvcalls-front.c | 91 
> > +++++++++++++++++++++++++++++++++++++++++++++
> >  drivers/xen/pvcalls-front.h |  8 ++++
> >  2 files changed, 99 insertions(+)
> >  create mode 100644 drivers/xen/pvcalls-front.h
> > 
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index fb08ebf..7933c73 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> 
> Shouldn't you include pvcalls-front.h?

Yes


> > @@ -40,9 +40,100 @@ struct pvcalls_bedata {
> >  
> >  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;
> > +
> > +   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;
> > +           src = (uint8_t *)&bedata->rsp[req_id];
> > +           src += sizeof(rsp->req_id);
> > +           dst = (uint8_t *)rsp;
> > +           dst += 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);
> > +
> > +           bedata->ring.rsp_cons++;
> > +           wake_up(&bedata->inflight_req);
> > +   }
> > +
> > +   RING_FINAL_CHECK_FOR_RESPONSES(&bedata->ring, more);
> > +   if (more)
> > +           goto again;
> 
> Wouldn't it make more sense to use wake_up() just once if there is any
> response pending and do the consuming loop outside the irq handler?

You are definitely right: it's far better to call wake_up() just once
after the consuming loop if there is any response pending. I'll do that.

However, I am not sure there is much to gain in moving the consuming
loop out of the irq handler: it's pretty short and doesn't call any long
running or sleeping functions.



 
> >     return IRQ_HANDLED;
> >  }
> >  
> > +int pvcalls_front_socket(struct socket *sock)
> > +{
> > +   struct pvcalls_bedata *bedata;
> > +   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);
> > +
> > +   spin_lock(&bedata->pvcallss_lock);
> > +   req_id = bedata->ring.req_prod_pvt & (RING_SIZE(&bedata->ring) - 1);
> > +   BUG_ON(req_id >= PVCALLS_NR_REQ_PER_RING);
> > +   if (RING_FULL(&bedata->ring) ||
> > +       READ_ONCE(bedata->rsp[req_id].req_id) != PVCALLS_INVALID_ID) {
> > +           spin_unlock(&bedata->pvcallss_lock);
> > +           return -EAGAIN;
> > +   }
> > +   req = RING_GET_REQUEST(&bedata->ring, req_id);
> > +   req->req_id = req_id;
> > +   req->cmd = PVCALLS_SOCKET;
> > +   req->u.socket.id = (uint64_t) sock;
> > +   req->u.socket.domain = AF_INET;
> > +   req->u.socket.type = SOCK_STREAM;
> > +   req->u.socket.protocol = 0;
> > +
> > +   bedata->ring.req_prod_pvt++;
> > +   RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&bedata->ring, notify);
> > +   spin_unlock(&bedata->pvcallss_lock);
> > +   if (notify)
> > +           notify_remote_via_irq(bedata->irq);
> > +
> > +   if (wait_event_interruptible(bedata->inflight_req,
> > +           READ_ONCE(bedata->rsp[req_id].req_id) == req_id) != 0)
> > +           return -EINTR;
> > +
> > +   ret = bedata->rsp[req_id].ret;
> > +   /* read ret, then set this rsp slot to be reused */
> > +   smp_mb();
> > +   WRITE_ONCE(bedata->rsp[req_id].req_id, PVCALLS_INVALID_ID);
> > +
> > +   return ret;
> > +}
> > +
> >  static const struct xenbus_device_id pvcalls_front_ids[] = {
> >     { "pvcalls" },
> >     { "" }
> > diff --git a/drivers/xen/pvcalls-front.h b/drivers/xen/pvcalls-front.h
> > new file mode 100644
> > index 0000000..b7dabed
> > --- /dev/null
> > +++ b/drivers/xen/pvcalls-front.h
> > @@ -0,0 +1,8 @@
> > +#ifndef __PVCALLS_FRONT_H__
> > +#define __PVCALLS_FRONT_H__
> > +
> > +#include <linux/net.h>
> > +
> > +int pvcalls_front_socket(struct socket *sock);
> > +
> > +#endif
> > 
> 

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