[PATCH RFC] xen/evtchn: Implement EVTCHNOP_send_imm as a companian to EVTCHNOP_send

[Andrew Cooper <andrew.cooper3@citrix.com>]

The event channel ABI passes a pointer to a subop-specific structure.  Some of
these structures however are smaller than the pointers passed in the ABI;
EVTCHNOP_send specifically passes a 4 byte evtchn port, using a 4 or 8 byte
pointer.

For translated guests (x86 HVM and all ARM), passing the port number directly
avoids a guest pagetable walk, which can be very expensive for Xen to perform.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Julien Grall <julien.grall@arm.com>
CC: Juergen Gross <jgross@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>

RFC: is it worth converting the close, unmask, reset (and possibly
expand_array) as well?
---
 xen/common/event_channel.c         |  4 ++++
 xen/include/public/event_channel.h | 13 ++++++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index c69f9db..79d8614 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1098,6 +1098,10 @@ long do_event_channel_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         break;
     }
 
+    case EVTCHNOP_send_imm:
+        rc = evtchn_send(current->domain, (unsigned long)arg.p);
+        break;
+
     case EVTCHNOP_status: {
         struct evtchn_status status;
         if ( copy_from_guest(&status, arg, 1) != 0 )
diff --git a/xen/include/public/event_channel.h b/xen/include/public/event_channel.h
index 44c549d..834787a 100644
--- a/xen/include/public/event_channel.h
+++ b/xen/include/public/event_channel.h
@@ -57,6 +57,9 @@
  * `
  * @cmd  == EVTCHNOP_* (event-channel operation).
  * @args == struct evtchn_* Operation-specific extra arguments (NULL if none).
+ *
+ * For @cmd with an _imm suffix, data are passed as an integer, rather than a
+ * pointer to a structure.
  */
 
 /* ` enum event_channel_op { // EVTCHNOP_* => struct evtchn_* */
@@ -64,7 +67,7 @@
 #define EVTCHNOP_bind_virq        1
 #define EVTCHNOP_bind_pirq        2
 #define EVTCHNOP_close            3
-#define EVTCHNOP_send             4
+#define EVTCHNOP_send             4 /* Deprecated.  Use EVTCHNOP_send_imm in preference. */
 #define EVTCHNOP_status           5
 #define EVTCHNOP_alloc_unbound    6
 #define EVTCHNOP_bind_ipi         7
@@ -74,6 +77,7 @@
 #define EVTCHNOP_init_control    11
 #define EVTCHNOP_expand_array    12
 #define EVTCHNOP_set_priority    13
+#define EVTCHNOP_send_imm        14
 /* ` } */
 
 typedef uint32_t evtchn_port_t;
@@ -186,8 +190,11 @@ struct evtchn_close {
 typedef struct evtchn_close evtchn_close_t;
 
 /*
- * EVTCHNOP_send: Send an event to the remote end of the channel whose local
- * endpoint is <port>.
+ * EVTCHNOP_send{,_imm}: Send an event to the remote end of the channel whose
+ * local endpoint is <port>.
+ *
+ * For EVTCHNOP_send, arg is a pointer to an evtchn_send_t.  For
+ * EVTCHNOP_send_imm, arg is the port directly.
  */
 struct evtchn_send {
     /* IN parameters. */
-- 
2.1.4


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