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Re: [PATCH V1 02/16] xen/ioreq: Make x86's IOREQ feature common





On 10/09/2020 21:21, Oleksandr Tyshchenko wrote:
+static bool hvm_wait_for_io(struct hvm_ioreq_vcpu *sv, ioreq_t *p)
+{
+    unsigned int prev_state = STATE_IOREQ_NONE;
+    unsigned int state = p->state;
+    uint64_t data = ~0;
+
+    smp_rmb();
+
+    /*
+     * The only reason we should see this condition be false is when an
+     * emulator dying races with I/O being requested.
+     */
+    while ( likely(state != STATE_IOREQ_NONE) )
+    {
+        if ( unlikely(state < prev_state) )
+        {
+            gdprintk(XENLOG_ERR, "Weird HVM ioreq state transition %u -> %u\n",
+                     prev_state, state);
+            sv->pending = false;
+            domain_crash(sv->vcpu->domain);
+            return false; /* bail */
+        }
+
+        switch ( prev_state = state )
+        {
+        case STATE_IORESP_READY: /* IORESP_READY -> NONE */
+            p->state = STATE_IOREQ_NONE;
+            data = p->data;
+            break;
+
+        case STATE_IOREQ_READY:  /* IOREQ_{READY,INPROCESS} -> IORESP_READY */
+        case STATE_IOREQ_INPROCESS:
+            wait_on_xen_event_channel(sv->ioreq_evtchn,
+                                      ({ state = p->state;
+                                         smp_rmb();
+                                         state != prev_state; }));
+            continue;

As I pointed out previously [1], this helper was implemented with the expectation that wait_on_xen_event_channel() will not return if the vCPU got rescheduled.

However, this assumption doesn't hold on Arm.

I can see two solution:
   1) Re-execute the caller
   2) Prevent an IOREQ to disappear until the loop finish.

@Paul any opinions?

Cheers,

[1] https://lore.kernel.org/xen-devel/6bfc3920-8f29-188c-cff4-2b99dabe166f@xxxxxxx/


--
Julien Grall



 


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