vpci: fix deferral of long operations

Current logic to handle long running operations is flawed because it
doesn't prevent the guest vcpu from running. Fix this by raising a
scheduler softirq when preemption is required, so that the do_softirq
call in the guest entry path performs a rescheduling. Also move the
call to vpci_process_pending into handle_hvm_io_completion, together
with the IOREQ code that handles pending IO instructions.

Note that a scheduler softirq is also raised when the long running
operation is queued in order to prevent the guest vcpu from resuming
execution.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/ioreq.c

@@ -85,9 +85,6 @@ bool hvm_io_pending(struct vcpu *v)
     struct hvm_ioreq_server *s;
     unsigned int id;
 
-    if ( has_vpci(d) && vpci_process_pending(v) )
-        return true;
-
     FOR_EACH_IOREQ_SERVER(d, id, s)
     {
         struct hvm_ioreq_vcpu *sv;
@@ -186,6 +183,12 @@ bool handle_hvm_io_completion(struct vcpu *v)
     enum hvm_io_completion io_completion;
     unsigned int id;
 
+    if ( has_vpci(d) && vpci_process_pending(v) )
+    {
+        raise_softirq(SCHEDULE_SOFTIRQ);
+        return false;
+    }
+
     FOR_EACH_IOREQ_SERVER(d, id, s)
     {
         struct hvm_ioreq_vcpu *sv;

xen/drivers/vpci/header.c

@@ -184,6 +184,12 @@ static void defer_map(struct domain *d, struct pci_dev *pdev,
     curr->vpci.mem = mem;
     curr->vpci.cmd = cmd;
     curr->vpci.rom_only = rom_only;
+    /*
+     * Raise a scheduler softirq in order to prevent the guest from resuming
+     * execution with pending mapping operations, to trigger the invocation
+     * of vpci_process_pending().
+     */
+    raise_softirq(SCHEDULE_SOFTIRQ);
 }
 
 static int modify_bars(const struct pci_dev *pdev, uint16_t cmd, bool rom_only)