[Xen-changelog] [qemu-xen-unstable] HVM: atomically access pointers in bufioreq handling

[patchbot@xxxxxxx]

commit 5cdde31eacdd288359746019ad05cac8ed5d9f70
Author:     Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
AuthorDate: Fri Aug 28 15:53:40 2015 +0100
Commit:     Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CommitDate: Fri Aug 28 15:53:40 2015 +0100

    HVM: atomically access pointers in bufioreq handling
    
    The number of slots per page being 511 (i.e. not a power of two) means
    that the (32-bit) read and write indexes going beyond 2^32 will likely
    disturb operation. The hypervisor side gets I/O req server creation
    extended so we can indicate that we're using suitable atomic accesses
    where needed, allowing it to atomically canonicalize both pointers when
    both have gone through at least one cycle.
    
    The Xen side counterpart (which is not a functional prereq to this
    change, albeit the intention is for Xen to assume default servers
    always use suitable atomic accesses) went in already (commit
    b7007bc6f9).
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
    ---
    v2: Adjust description.
---
 i386-dm/helper2.c |   24 ++++++++++++++++--------
 1 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/i386-dm/helper2.c b/i386-dm/helper2.c
index 63a938b..ede3105 100644
--- a/i386-dm/helper2.c
+++ b/i386-dm/helper2.c
@@ -493,10 +493,19 @@ static int __handle_buffered_iopage(CPUState *env)
 
     memset(&req, 0x00, sizeof(req));
 
-    while (buffered_io_page->read_pointer !=
-           buffered_io_page->write_pointer) {
-        buf_req = &buffered_io_page->buf_ioreq[
-            buffered_io_page->read_pointer % IOREQ_BUFFER_SLOT_NUM];
+    for (;;) {
+        uint32_t rdptr = buffered_io_page->read_pointer, wrptr;
+
+        xen_rmb();
+        wrptr = buffered_io_page->write_pointer;
+        xen_rmb();
+        if (rdptr != buffered_io_page->read_pointer) {
+            continue;
+        }
+        if (rdptr == wrptr) {
+            break;
+        }
+        buf_req = &buffered_io_page->buf_ioreq[rdptr % IOREQ_BUFFER_SLOT_NUM];
         req.size = 1UL << buf_req->size;
         req.count = 1;
         req.addr = buf_req->addr;
@@ -508,15 +517,14 @@ static int __handle_buffered_iopage(CPUState *env)
         req.data_is_ptr = 0;
         qw = (req.size == 8);
         if (qw) {
-            buf_req = &buffered_io_page->buf_ioreq[
-                (buffered_io_page->read_pointer+1) % IOREQ_BUFFER_SLOT_NUM];
+            buf_req = &buffered_io_page->buf_ioreq[(rdptr + 1) %
+                                                   IOREQ_BUFFER_SLOT_NUM];
             req.data |= ((uint64_t)buf_req->data) << 32;
         }
 
         __handle_ioreq(env, &req);
 
-        xen_mb();
-        buffered_io_page->read_pointer += qw ? 2 : 1;
+        __sync_fetch_and_add(&buffered_io_page->read_pointer, qw + 1);
     }
 
     return req.count;
--
generated by git-patchbot for /home/xen/git/qemu-xen-unstable.git

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