[Xen-devel] [PATCH 1/6] xenpaging: correct dropping pages to avoid full ring buffer

[Olaf Hering <olaf@xxxxxxxxx>]

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. A simple fix is to take 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>

---
 tools/xenpaging/xenpaging.c |   22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

--- xen-unstable.hg-4.1.22764.orig/tools/xenpaging/xenpaging.c
+++ xen-unstable.hg-4.1.22764/tools/xenpaging/xenpaging.c
@@ -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 */


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