Re: [Qemu-devel] [PATCH v2 2/9] xen: import ring.h from xen

[Stefano Stabellini <sstabellini@kernel.org>]

On Wed, 15 Mar 2017, Greg Kurz wrote:
> On Mon, 13 Mar 2017 16:55:53 -0700
> Stefano Stabellini <sstabellini@kernel.org> wrote:
> 
> > Do not use the ring.h header installed on the system. Instead, import
> > the header into the QEMU codebase. This avoids problems when QEMU is
> > built against a Xen version too old to provide all the ring macros.
> > 
> 
> What kind of problems ?

The FLEX macros are only available in Xen 4.9+ (still unreleased).
However, aside from these macros, the Xen 9pfs frontends and backends
could work fine on any Xen releases. In fact, the Xen public/io headers
are only provided as reference.


> > Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
> > CC: anthony.perard@citrix.com
> > CC: jgross@suse.com
> > ---
> > NB: The new macros have not been committed to Xen yet. Do not apply this
> > patch until they do.
> 
> Why ? Does this break compat with older Xen ?

No, it does not break compatibility. But I think it is a good idea to
commit the header to QEMU only after the corresponding Xen header has
been accepted. I don't want the two to diverge.


> > ---
> > ---
> >  hw/block/xen_blkif.h     |   2 +-
> >  hw/usb/xen-usb.c         |   2 +-
> >  include/hw/xen/io/ring.h | 455 +++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 457 insertions(+), 2 deletions(-)
> >  create mode 100644 include/hw/xen/io/ring.h
> > 
> > diff --git a/hw/block/xen_blkif.h b/hw/block/xen_blkif.h
> > index 3300b6f..3e6e1ea 100644
> > --- a/hw/block/xen_blkif.h
> > +++ b/hw/block/xen_blkif.h
> > @@ -1,7 +1,7 @@
> >  #ifndef XEN_BLKIF_H
> >  #define XEN_BLKIF_H
> >  
> > -#include <xen/io/ring.h>
> > +#include "hw/xen/io/ring.h"
> >  #include <xen/io/blkif.h>
> >  #include <xen/io/protocols.h>
> >  
> > diff --git a/hw/usb/xen-usb.c b/hw/usb/xen-usb.c
> > index 8e676e6..370b3d9 100644
> > --- a/hw/usb/xen-usb.c
> > +++ b/hw/usb/xen-usb.c
> > @@ -33,7 +33,7 @@
> >  #include "qapi/qmp/qint.h"
> >  #include "qapi/qmp/qstring.h"
> >  
> > -#include <xen/io/ring.h>
> > +#include "hw/xen/io/ring.h"
> >  #include <xen/io/usbif.h>
> >  
> >  /*
> > diff --git a/include/hw/xen/io/ring.h b/include/hw/xen/io/ring.h
> > new file mode 100644
> > index 0000000..cf01fc3
> > --- /dev/null
> > +++ b/include/hw/xen/io/ring.h
> > @@ -0,0 +1,455 @@
> > +/******************************************************************************
> > + * ring.h
> > + * 
> > + * Shared producer-consumer ring macros.
> > + *
> > + * Permission is hereby granted, free of charge, to any person obtaining a copy
> > + * of this software and associated documentation files (the "Software"), to
> > + * deal in the Software without restriction, including without limitation the
> > + * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
> > + * sell copies of the Software, and to permit persons to whom the Software is
> > + * furnished to do so, subject to the following conditions:
> > + *
> > + * The above copyright notice and this permission notice shall be included in
> > + * all copies or substantial portions of the Software.
> > + *
> > + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> > + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> > + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
> > + * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> > + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> > + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
> > + * DEALINGS IN THE SOFTWARE.
> > + *
> > + * Tim Deegan and Andrew Warfield November 2004.
> > + */
> > +
> > +#ifndef __XEN_PUBLIC_IO_RING_H__
> > +#define __XEN_PUBLIC_IO_RING_H__
> > +
> > +#if __XEN_INTERFACE_VERSION__ < 0x00030208
> > +#define xen_mb()  mb()
> > +#define xen_rmb() rmb()
> > +#define xen_wmb() wmb()
> > +#endif
> > +
> > +typedef unsigned int RING_IDX;
> > +
> > +/* Round a 32-bit unsigned constant down to the nearest power of two. */
> > +#define __RD2(_x)  (((_x) & 0x00000002) ? 0x2                  : ((_x) & 0x1))
> > +#define __RD4(_x)  (((_x) & 0x0000000c) ? __RD2((_x)>>2)<<2    : __RD2(_x))
> > +#define __RD8(_x)  (((_x) & 0x000000f0) ? __RD4((_x)>>4)<<4    : __RD4(_x))
> > +#define __RD16(_x) (((_x) & 0x0000ff00) ? __RD8((_x)>>8)<<8    : __RD8(_x))
> > +#define __RD32(_x) (((_x) & 0xffff0000) ? __RD16((_x)>>16)<<16 : __RD16(_x))
> > +
> > +/*
> > + * Calculate size of a shared ring, given the total available space for the
> > + * ring and indexes (_sz), and the name tag of the request/response structure.
> > + * A ring contains as many entries as will fit, rounded down to the nearest 
> > + * power of two (so we can mask with (size-1) to loop around).
> > + */
> > +#define __CONST_RING_SIZE(_s, _sz) \
> > +    (__RD32(((_sz) - offsetof(struct _s##_sring, ring)) / \
> > +	    sizeof(((struct _s##_sring *)0)->ring[0])))
> > +/*
> > + * The same for passing in an actual pointer instead of a name tag.
> > + */
> > +#define __RING_SIZE(_s, _sz) \
> > +    (__RD32(((_sz) - (long)(_s)->ring + (long)(_s)) / sizeof((_s)->ring[0])))
> > +
> > +/*
> > + * Macros to make the correct C datatypes for a new kind of ring.
> > + * 
> > + * To make a new ring datatype, you need to have two message structures,
> > + * let's say request_t, and response_t already defined.
> > + *
> > + * In a header where you want the ring datatype declared, you then do:
> > + *
> > + *     DEFINE_RING_TYPES(mytag, request_t, response_t);
> > + *
> > + * These expand out to give you a set of types, as you can see below.
> > + * The most important of these are:
> > + * 
> > + *     mytag_sring_t      - The shared ring.
> > + *     mytag_front_ring_t - The 'front' half of the ring.
> > + *     mytag_back_ring_t  - The 'back' half of the ring.
> > + *
> > + * To initialize a ring in your code you need to know the location and size
> > + * of the shared memory area (PAGE_SIZE, for instance). To initialise
> > + * the front half:
> > + *
> > + *     mytag_front_ring_t front_ring;
> > + *     SHARED_RING_INIT((mytag_sring_t *)shared_page);
> > + *     FRONT_RING_INIT(&front_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
> > + *
> > + * Initializing the back follows similarly (note that only the front
> > + * initializes the shared ring):
> > + *
> > + *     mytag_back_ring_t back_ring;
> > + *     BACK_RING_INIT(&back_ring, (mytag_sring_t *)shared_page, PAGE_SIZE);
> > + */
> > +
> > +#define DEFINE_RING_TYPES(__name, __req_t, __rsp_t)                     \
> > +                                                                        \
> > +/* Shared ring entry */                                                 \
> > +union __name##_sring_entry {                                            \
> > +    __req_t req;                                                        \
> > +    __rsp_t rsp;                                                        \
> > +};                                                                      \
> > +                                                                        \
> > +/* Shared ring page */                                                  \
> > +struct __name##_sring {                                                 \
> > +    RING_IDX req_prod, req_event;                                       \
> > +    RING_IDX rsp_prod, rsp_event;                                       \
> > +    union {                                                             \
> > +        struct {                                                        \
> > +            uint8_t smartpoll_active;                                   \
> > +        } netif;                                                        \
> > +        struct {                                                        \
> > +            uint8_t msg;                                                \
> > +        } tapif_user;                                                   \
> > +        uint8_t pvt_pad[4];                                             \
> > +    } pvt;                                                              \
> > +    uint8_t __pad[44];                                                  \
> > +    union __name##_sring_entry ring[1]; /* variable-length */           \
> > +};                                                                      \
> > +                                                                        \
> > +/* "Front" end's private variables */                                   \
> > +struct __name##_front_ring {                                            \
> > +    RING_IDX req_prod_pvt;                                              \
> > +    RING_IDX rsp_cons;                                                  \
> > +    unsigned int nr_ents;                                               \
> > +    struct __name##_sring *sring;                                       \
> > +};                                                                      \
> > +                                                                        \
> > +/* "Back" end's private variables */                                    \
> > +struct __name##_back_ring {                                             \
> > +    RING_IDX rsp_prod_pvt;                                              \
> > +    RING_IDX req_cons;                                                  \
> > +    unsigned int nr_ents;                                               \
> > +    struct __name##_sring *sring;                                       \
> > +};                                                                      \
> > +                                                                        \
> > +/* Syntactic sugar */                                                   \
> > +typedef struct __name##_sring __name##_sring_t;                         \
> > +typedef struct __name##_front_ring __name##_front_ring_t;               \
> > +typedef struct __name##_back_ring __name##_back_ring_t
> > +
> > +/*
> > + * Macros for manipulating rings.
> > + * 
> > + * FRONT_RING_whatever works on the "front end" of a ring: here 
> > + * requests are pushed on to the ring and responses taken off it.
> > + * 
> > + * BACK_RING_whatever works on the "back end" of a ring: here 
> > + * requests are taken off the ring and responses put on.
> > + * 
> > + * N.B. these macros do NO INTERLOCKS OR FLOW CONTROL. 
> > + * This is OK in 1-for-1 request-response situations where the 
> > + * requestor (front end) never has more than RING_SIZE()-1
> > + * outstanding requests.
> > + */
> > +
> > +/* Initialising empty rings */
> > +#define SHARED_RING_INIT(_s) do {                                       \
> > +    (_s)->req_prod  = (_s)->rsp_prod  = 0;                              \
> > +    (_s)->req_event = (_s)->rsp_event = 1;                              \
> > +    (void)memset((_s)->pvt.pvt_pad, 0, sizeof((_s)->pvt.pvt_pad));      \
> > +    (void)memset((_s)->__pad, 0, sizeof((_s)->__pad));                  \
> > +} while(0)
> > +
> > +#define FRONT_RING_INIT(_r, _s, __size) do {                            \
> > +    (_r)->req_prod_pvt = 0;                                             \
> > +    (_r)->rsp_cons = 0;                                                 \
> > +    (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
> > +    (_r)->sring = (_s);                                                 \
> > +} while (0)
> > +
> > +#define BACK_RING_INIT(_r, _s, __size) do {                             \
> > +    (_r)->rsp_prod_pvt = 0;                                             \
> > +    (_r)->req_cons = 0;                                                 \
> > +    (_r)->nr_ents = __RING_SIZE(_s, __size);                            \
> > +    (_r)->sring = (_s);                                                 \
> > +} while (0)
> > +
> > +/* How big is this ring? */
> > +#define RING_SIZE(_r)                                                   \
> > +    ((_r)->nr_ents)
> > +
> > +/* Number of free requests (for use on front side only). */
> > +#define RING_FREE_REQUESTS(_r)                                          \
> > +    (RING_SIZE(_r) - ((_r)->req_prod_pvt - (_r)->rsp_cons))
> > +
> > +/* Test if there is an empty slot available on the front ring.
> > + * (This is only meaningful from the front. )
> > + */
> > +#define RING_FULL(_r)                                                   \
> > +    (RING_FREE_REQUESTS(_r) == 0)
> > +
> > +/* Test if there are outstanding messages to be processed on a ring. */
> > +#define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
> > +    ((_r)->sring->rsp_prod - (_r)->rsp_cons)
> > +
> > +#ifdef __GNUC__
> > +#define RING_HAS_UNCONSUMED_REQUESTS(_r) ({                             \
> > +    unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;          \
> > +    unsigned int rsp = RING_SIZE(_r) -                                  \
> > +        ((_r)->req_cons - (_r)->rsp_prod_pvt);                          \
> > +    req < rsp ? req : rsp;                                              \
> > +})
> > +#else
> > +/* Same as above, but without the nice GCC ({ ... }) syntax. */
> > +#define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
> > +    ((((_r)->sring->req_prod - (_r)->req_cons) <                        \
> > +      (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt))) ?        \
> > +     ((_r)->sring->req_prod - (_r)->req_cons) :                         \
> > +     (RING_SIZE(_r) - ((_r)->req_cons - (_r)->rsp_prod_pvt)))
> > +#endif
> > +
> > +/* Direct access to individual ring elements, by index. */
> > +#define RING_GET_REQUEST(_r, _idx)                                      \
> > +    (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
> > +
> > +/*
> > + * Get a local copy of a request.
> > + *
> > + * Use this in preference to RING_GET_REQUEST() so all processing is
> > + * done on a local copy that cannot be modified by the other end.
> > + *
> > + * Note that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 may cause this
> > + * to be ineffective where _req is a struct which consists of only bitfields.
> > + */
> > +#define RING_COPY_REQUEST(_r, _idx, _req) do {				\
> > +	/* Use volatile to force the copy into _req. */			\
> > +	*(_req) = *(volatile typeof(_req))RING_GET_REQUEST(_r, _idx);	\
> > +} while (0)
> > +
> > +#define RING_GET_RESPONSE(_r, _idx)                                     \
> > +    (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
> > +
> > +/* Loop termination condition: Would the specified index overflow the ring? */
> > +#define RING_REQUEST_CONS_OVERFLOW(_r, _cons)                           \
> > +    (((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
> > +
> > +/* Ill-behaved frontend determination: Can there be this many requests? */
> > +#define RING_REQUEST_PROD_OVERFLOW(_r, _prod)                           \
> > +    (((_prod) - (_r)->rsp_prod_pvt) > RING_SIZE(_r))
> > +
> > +#define RING_PUSH_REQUESTS(_r) do {                                     \
> > +    xen_wmb(); /* back sees requests /before/ updated producer index */ \
> > +    (_r)->sring->req_prod = (_r)->req_prod_pvt;                         \
> > +} while (0)
> > +
> > +#define RING_PUSH_RESPONSES(_r) do {                                    \
> > +    xen_wmb(); /* front sees resps /before/ updated producer index */   \
> > +    (_r)->sring->rsp_prod = (_r)->rsp_prod_pvt;                         \
> > +} while (0)
> > +
> > +/*
> > + * Notification hold-off (req_event and rsp_event):
> > + * 
> > + * When queueing requests or responses on a shared ring, it may not always be
> > + * necessary to notify the remote end. For example, if requests are in flight
> > + * in a backend, the front may be able to queue further requests without
> > + * notifying the back (if the back checks for new requests when it queues
> > + * responses).
> > + * 
> > + * When enqueuing requests or responses:
> > + * 
> > + *  Use RING_PUSH_{REQUESTS,RESPONSES}_AND_CHECK_NOTIFY(). The second argument
> > + *  is a boolean return value. True indicates that the receiver requires an
> > + *  asynchronous notification.
> > + * 
> > + * After dequeuing requests or responses (before sleeping the connection):
> > + * 
> > + *  Use RING_FINAL_CHECK_FOR_REQUESTS() or RING_FINAL_CHECK_FOR_RESPONSES().
> > + *  The second argument is a boolean return value. True indicates that there
> > + *  are pending messages on the ring (i.e., the connection should not be put
> > + *  to sleep).
> > + * 
> > + *  These macros will set the req_event/rsp_event field to trigger a
> > + *  notification on the very next message that is enqueued. If you want to
> > + *  create batches of work (i.e., only receive a notification after several
> > + *  messages have been enqueued) then you will need to create a customised
> > + *  version of the FINAL_CHECK macro in your own code, which sets the event
> > + *  field appropriately.
> > + */
> > +
> > +#define RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(_r, _notify) do {           \
> > +    RING_IDX __old = (_r)->sring->req_prod;                             \
> > +    RING_IDX __new = (_r)->req_prod_pvt;                                \
> > +    xen_wmb(); /* back sees requests /before/ updated producer index */ \
> > +    (_r)->sring->req_prod = __new;                                      \
> > +    xen_mb(); /* back sees new requests /before/ we check req_event */  \
> > +    (_notify) = ((RING_IDX)(__new - (_r)->sring->req_event) <           \
> > +                 (RING_IDX)(__new - __old));                            \
> > +} while (0)
> > +
> > +#define RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(_r, _notify) do {          \
> > +    RING_IDX __old = (_r)->sring->rsp_prod;                             \
> > +    RING_IDX __new = (_r)->rsp_prod_pvt;                                \
> > +    xen_wmb(); /* front sees resps /before/ updated producer index */   \
> > +    (_r)->sring->rsp_prod = __new;                                      \
> > +    xen_mb(); /* front sees new resps /before/ we check rsp_event */    \
> > +    (_notify) = ((RING_IDX)(__new - (_r)->sring->rsp_event) <           \
> > +                 (RING_IDX)(__new - __old));                            \
> > +} while (0)
> > +
> > +#define RING_FINAL_CHECK_FOR_REQUESTS(_r, _work_to_do) do {             \
> > +    (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
> > +    if (_work_to_do) break;                                             \
> > +    (_r)->sring->req_event = (_r)->req_cons + 1;                        \
> > +    xen_mb();                                                           \
> > +    (_work_to_do) = RING_HAS_UNCONSUMED_REQUESTS(_r);                   \
> > +} while (0)
> > +
> > +#define RING_FINAL_CHECK_FOR_RESPONSES(_r, _work_to_do) do {            \
> > +    (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
> > +    if (_work_to_do) break;                                             \
> > +    (_r)->sring->rsp_event = (_r)->rsp_cons + 1;                        \
> > +    xen_mb();                                                           \
> > +    (_work_to_do) = RING_HAS_UNCONSUMED_RESPONSES(_r);                  \
> > +} while (0)
> > +
> > +
> > +/*
> > + * DEFINE_XEN_FLEX_RING_AND_INTF defines two monodirectional rings and
> > + * functions to check if there is data on the ring, and to read and
> > + * write to them.
> > + *
> > + * DEFINE_XEN_FLEX_RING is similar to DEFINE_XEN_FLEX_RING_AND_INTF, but
> > + * does not define the indexes page. As different protocols can have
> > + * extensions to the basic format, this macro allow them to define their
> > + * own struct.
> > + *
> > + * XEN_FLEX_RING_SIZE
> > + *   Convenience macro to calculate the size of one of the two rings
> > + *   from the overall order.
> > + *
> > + * $NAME_mask
> > + *   Function to apply the size mask to an index, to reduce the index
> > + *   within the range [0-size].
> > + *
> > + * $NAME_read_packet
> > + *   Function to read data from the ring. The amount of data to read is
> > + *   specified by the "size" argument.
> > + *
> > + * $NAME_write_packet
> > + *   Function to write data to the ring. The amount of data to write is
> > + *   specified by the "size" argument.
> > + *
> > + * $NAME_get_ring_ptr
> > + *   Convenience function that returns a pointer to read/write to the
> > + *   ring at the right location.
> > + *
> > + * $NAME_data_intf
> > + *   Indexes page, shared between frontend and backend. It also
> > + *   contains the array of grant refs.
> > + *
> > + * $NAME_queued
> > + *   Function to calculate how many bytes are currently on the ring,
> > + *   ready to be read. It can also be used to calculate how much free
> > + *   space is currently on the ring (ring_size - $NAME_queued()).
> > + */
> > +#define XEN_FLEX_RING_SIZE(order)                                             \
> > +    (1UL << (order + PAGE_SHIFT - 1))
> > +
> > +#define DEFINE_XEN_FLEX_RING_AND_INTF(name)                                   \
> > +struct name##_data_intf {                                                     \
> > +    RING_IDX in_cons, in_prod;                                                \
> > +                                                                              \
> > +    uint8_t pad1[56];                                                         \
> > +                                                                              \
> > +    RING_IDX out_cons, out_prod;                                              \
> > +                                                                              \
> > +    uint8_t pad2[56];                                                         \
> > +                                                                              \
> > +    RING_IDX ring_order;                                                      \
> > +    grant_ref_t ref[];                                                        \
> > +};                                                                            \
> > +DEFINE_XEN_FLEX_RING(name);
> > +
> > +#define DEFINE_XEN_FLEX_RING(name)                                            \
> > +static inline RING_IDX name##_mask(RING_IDX idx, RING_IDX ring_size)          \
> > +{                                                                             \
> > +    return (idx & (ring_size - 1));                                           \
> > +}                                                                             \
> > +                                                                              \
> > +static inline RING_IDX name##_mask_order(RING_IDX idx, RING_IDX ring_order)   \
> > +{                                                                             \
> > +    return (idx & (XEN_FLEX_RING_SIZE(ring_order) - 1));                      \
> > +}                                                                             \
> > +                                                                              \
> > +static inline unsigned char* name##_get_ring_ptr(unsigned char *buf,          \
> > +                                                 RING_IDX idx,                \
> > +                                                 RING_IDX ring_order)         \
> > +{                                                                             \
> > +    return buf + name##_mask_order(idx, ring_order);                          \
> > +}                                                                             \
> > +                                                                              \
> > +static inline void name##_read_packet(const unsigned char *buf,               \
> > +        RING_IDX masked_prod, RING_IDX *masked_cons,                          \
> > +        RING_IDX ring_size, void *opaque, size_t size) {                      \
> > +    if (*masked_cons < masked_prod ||                                         \
> > +            size <= ring_size - *masked_cons) {                               \
> > +        memcpy(opaque, buf + *masked_cons, size);                             \
> > +    } else {                                                                  \
> > +        memcpy(opaque, buf + *masked_cons, ring_size - *masked_cons);         \
> > +        memcpy((unsigned char *)opaque + ring_size - *masked_cons, buf,       \
> > +                size - (ring_size - *masked_cons));                           \
> > +    }                                                                         \
> > +    *masked_cons = name##_mask(*masked_cons + size, ring_size);               \
> > +}                                                                             \
> > +                                                                              \
> > +static inline void name##_write_packet(unsigned char *buf,                    \
> > +        RING_IDX *masked_prod, RING_IDX masked_cons,                          \
> > +        RING_IDX ring_size, const void *opaque, size_t size) {                \
> > +    if (*masked_prod < masked_cons ||                                         \
> > +        size <= ring_size - *masked_prod) {                                   \
> > +        memcpy(buf + *masked_prod, opaque, size);                             \
> > +    } else {                                                                  \
> > +        memcpy(buf + *masked_prod, opaque, ring_size - *masked_prod);         \
> > +        memcpy(buf, (unsigned char *)opaque + (ring_size - *masked_prod),     \
> > +                size - (ring_size - *masked_prod));                           \
> > +    }                                                                         \
> > +    *masked_prod = name##_mask(*masked_prod + size, ring_size);               \
> > +}                                                                             \
> > +                                                                              \
> > +struct name##_data {                                                          \
> > +    unsigned char *in; /* half of the allocation */                           \
> > +    unsigned char *out; /* half of the allocation */                          \
> > +};                                                                            \
> > +                                                                              \
> > +                                                                              \
> > +static inline RING_IDX name##_queued(RING_IDX prod,                           \
> > +        RING_IDX cons, RING_IDX ring_size)                                    \
> > +{                                                                             \
> > +    RING_IDX size;                                                            \
> > +                                                                              \
> > +    if (prod == cons)                                                         \
> > +        return 0;                                                             \
> > +                                                                              \
> > +    prod = name##_mask(prod, ring_size);                                      \
> > +    cons = name##_mask(cons, ring_size);                                      \
> > +                                                                              \
> > +    if (prod == cons)                                                         \
> > +        return ring_size;                                                     \
> > +                                                                              \
> > +    if (prod > cons)                                                          \
> > +        size = prod - cons;                                                   \
> > +    else                                                                      \
> > +        size = ring_size - (cons - prod);                                     \
> > +    return size;                                                              \
> > +};
> > +
> > +#endif /* __XEN_PUBLIC_IO_RING_H__ */
> > +
> > +/*
> > + * Local variables:
> > + * mode: C
> > + * c-file-style: "BSD"
> > + * c-basic-offset: 4
> > + * tab-width: 4
> > + * indent-tabs-mode: nil
> > + * End:
> > + */
> 
>