[Qemu-devel] [BUG] 50MB/min logspam: dma: unregistered DMA channel used nchan=0 dma_pos=0 dma_len=1

[Philipp Hahn]

Hello,

a Xen-4.1-3 host system filled its log file with the message
 dma: unregistered DMA channel used nchan=0 dma_pos=0 dma_len=1

The happened two times was a Windows Server 2003 with GPLPV 0.11.0.372:
one VM was migrated to that host and one was directly started there.
After a restart of the VM the message no longer gets printed.

Looking at both xen-4.1.3/tools/ioemu-qemu-xen/hw/dma.c and
qemu-git/hw/dma/i8257.c the message is printed by dma_phony_handler(),
which is the default is no other transfer_handler is registered.

The function gets run through DMA_run_bh -> DMA_run -> channel_run.
In DMA_run() the code checks both the 'mask' and 'status' registers:
383   if ((0 == (d->mask & mask)) && (0 != (d->status & (mask << 4)))) {
384       channel_run (icont, ichan);
385       rearm = 1;
386    }

So the messages is printed with maximum frequency, as the BH gets
executed each time leading to massive log spam.

Looking further I analyzed the following case, which *might* be responsible:

While 'mask' is included in the VMStateDescription, 'status' is not.
Before the conversion to VMSTATE* the code was commented out from day-one:
512     /* qemu_put_8s (f, &d->status); */
540     /* qemu_get_8s (f, &d->status); */

1. Might it be that 'status' is uninitialized after migration?
2. Has someone other seen that message?
3. Could the rate of the message please be limited? See attached patch.
4. Some other ideas or comments?

Sincerely
Philipp
-- 
Philipp Hahn
Open Source Software Engineer

Univention GmbH
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0001-hw-dma-Print-error-message-only-once.patch
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