Restore HVM_OP hypercall continuation (partial revert of ae20ccf)

Commit ae20ccf removed the hypercall continuation logic from the end
of do_hvm_op(), claiming:

"This patch removes the need for handling HVMOP restarts, so that
infrastructure is removed."

That turns out to be false.  The removal of HVMOP_set_mem_type removed
the need to store a start iteration value in the hypercall
continuation, but a grep through hvm.c for ERESTART turns up at least
two places where do_hvm_op() may still need a hypercall continuation:

 * HVMOP_set_hvm_param can return -ERESTART when setting
HVM_PARAM_IDENT_PT in the event that it fails to acquire the domctl
lock

 * HVMOP_flush_tlbs can return -ERESTART if several vcpus call it at
   the same time

In both cases, a simple restart (with no stored iteration information)
is necessary.

Add a check for -ERESTART again, along with a comment at the top of
the function regarding the lack of decoding any information from the
op value.

Reported-by: Xudong Hao <xudong.hao@intel.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 81691e275326a2b39d89bd0af7bea2572e8fd246..273bcff9ba6b5bf17dc674f6a6b54ff550c8bd0e 100644 (file)
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4544,6 +4544,13 @@ long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     long rc = 0;
 
+    /*
+     * NB: hvm_op can be part of a restarted hypercall; but at the
+     * moment the only hypercalls which do continuations don't need to
+     * store any iteration information (since they're just re-trying
+     * the acquisition of a lock).
+     */
+
     switch ( op )
     {
     case HVMOP_set_evtchn_upcall_vector:
@@ -4636,6 +4643,10 @@ long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE_PARAM(void) arg)
     }
     }
 
+    if ( rc == -ERESTART )
+        rc = hypercall_create_continuation(__HYPERVISOR_hvm_op, "lh",
+                                           op, arg);
+
     return rc;
 }