[PATCH 01 of 12] xenpaging: correct dropping of pages to avoid full ring buffer

[Olaf Hering <olaf@aepfle.de>]

# HG changeset patch
# User Olaf Hering <olaf@aepfle.de>
# Date 1307437209 -7200
# Node ID 6b8446bf4e5fbfa93169ec2509364c0fde74beca
# Parent  c231a26a29327aa3c737170e04c738289be2d309
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@aepfle.de>

diff -r c231a26a2932 -r 6b8446bf4e5f tools/xenpaging/xenpaging.c
--- a/tools/xenpaging/xenpaging.c	Mon Jun 06 09:56:08 2011 +0100
+++ b/tools/xenpaging/xenpaging.c	Tue Jun 07 11:00:09 2011 +0200
@@ -653,19 +653,19 @@ 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 */