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[Bug 1859359] Re: xHCI and event ring handling

From: Benjamin David Lunt
Subject: [Bug 1859359] Re: xHCI and event ring handling
Date: Tue, 14 Jan 2020 00:44:18 -0000

Hi Hector, guys,

I think I have found my/the error:

xHCI, version 1.0, Page 136:
"Note: The detection of a Cycle bit mismatch in an Event TRB processed 
 by software indicates the location of the xHC Event Ring Enqueue Pointer 
 and that the Event Ring is empty. Software shall write the ERDP with the
 address of this TRB to indicate that it has processed all Events in the

It does not state to advance the Consumer's internal Dequeue pointer.  
Just the register is mentioned.

This is my error.  I thought that it implied to advance the Consumer's
internal Dequeue Pointer as well.  (Implied being the big word here)

Sorry for the bother.  It was my error.  It took me a bit of (re)reading
and a little more work to find that it was/is indeed my error.

Sorry and thank you for your time,

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  xHCI and event ring handling

Status in QEMU:

Bug description:
  I believe that the Event Ring handling in QEMU is not correct.  For
  example, an Event Ring may have multiple segments.  However, the code
  in xhci_write_event()
  xhci.c;hb=HEAD#l645), starting with line 668, seems to only support a
  single segment.

  Also, QEMU is sending a spurious interrupt after the Guest writes to
  the ERDP register due to the fact that the address written does not
  match the current index.  This is because the index is incremented
  after sending the event.  The xHCI specification states in section "When software finishes processing an Event TRB, it will
  write the address of that Event TRB to the ERDP."

  Since xhci_write_event() has already incremented the index pointer
  (intr->er_ep_idx), the check at line 3098
  xhci.c;hb=HEAD#l3090) no longer is valid.

  I have not studied QEMU's code enough yet to offer a patch.  However,
  this should be a simple fix.

  if (intr->er_ep_idx >= intr->er_table[intr->er_segment].er_size) {
    intr->er_ep_idx = 0;
    if (intr->er_segment >= intr->er_table_size) {
      intr->er_segment = 0;
      intr->er_pcs = !intr->er_pcs;

  Being sure to incorporate this new segment member into the above code
  (possibly as shown) as well as change the lines at 665 to use the new
  segment member of the structure, and of course in the initialization
  portion of the event ring.

  As for the spurious interrupt at line 3101, a new member will need to
  be added to the code to keep track of the last inserted ED (TRB) into
  the Event Ring and then of course checking against this new member,
  not the now newly incremented member.

  I have sent an email to the author listed at the top of the file as
  well, not sure if this is proper bug reporting etiquette or not.

  Thank you.

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