[Xen-devel] [PATCH 10 of 22] xenpaging: correct dropping of pages to avoid full ring buffer

[Olaf Hering <olaf@xxxxxxxxx>]

# HG changeset patch
# User Olaf Hering <olaf@xxxxxxxxx>
# Date 1307695634 -7200
# Node ID 1de8de108d152fe915fc7f78044c406fed872bca
# Parent  9c42376aac05dc21a617d1d9fd62037cb8a9700d
xenpaging: correct dropping of pages to avoid full ring buffer

Doing a one-way channel from Xen to xenpaging is not possible with the
current ring buffer implementation. xenpaging uses the mem_event ring
buffer, which expects request/response pairs to make progress. The
previous patch, which tried to establish a one-way communication from
Xen to xenpaging, stalled the guest once the buffer was filled up with
requests. Correct page-dropping by taking the slow path and let
p2m_mem_paging_resume() consume the response from xenpaging. This makes
room for yet another request/response pair and avoids hanging guests.

Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>

diff -r 9c42376aac05 -r 1de8de108d15 tools/xenpaging/xenpaging.c
--- a/tools/xenpaging/xenpaging.c       Fri Jun 10 10:47:12 2011 +0200
+++ b/tools/xenpaging/xenpaging.c       Fri Jun 10 10:47:14 2011 +0200
@@ -690,19 +690,19 @@ int main(int argc, char *argv[])
                         ERROR("Error populating page");
                         goto out;
                     }
+                }
 
-                    /* Prepare the response */
-                    rsp.gfn = req.gfn;
-                    rsp.p2mt = req.p2mt;
-                    rsp.vcpu_id = req.vcpu_id;
-                    rsp.flags = req.flags;
+                /* Prepare the response */
+                rsp.gfn = req.gfn;
+                rsp.p2mt = req.p2mt;
+                rsp.vcpu_id = req.vcpu_id;
+                rsp.flags = req.flags;
 
-                    rc = xenpaging_resume_page(paging, &rsp, 1);
-                    if ( rc != 0 )
-                    {
-                        ERROR("Error resuming page");
-                        goto out;
-                    }
+                rc = xenpaging_resume_page(paging, &rsp, 1);
+                if ( rc != 0 )
+                {
+                    ERROR("Error resuming page");
+                    goto out;
                 }
 
                 /* Evict a new page to replace the one we just paged in */

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