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Date: Fri, 23 Mar 2012 11:02:30 +0000
From: Richard Davies <richard@arachsys.com>
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Subject: Re: [Qemu-devel] [PATCH] block: add the support to drain throttled
 requests
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Chris Webb <chris@arachsys.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org, Zhi Yong Wu <zwu.kernel@gmail.com>, Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>, Paolo Bonzini <pbonzini@redhat.com>

Stefan Hajnoczi wrote:
> > Hi. We were producing the IDE assert()s and deadlocks with linux kernels.
> > Although I believe the same symptoms exist on windows, I haven't actually
> > tested it myself. Typically they would show up in the 16-bit bootloader
> > code, even before the 32-bit OS has started.
>
> Okay, that makes sense.  Bootloaders and the BIOS may use the simplest
> driver interface - which may be PIO in the case.  I asked because the
> IDE DMA code path should work with I/O throttling and Windows is known
> for sometimes falling back to the PIO code path when some heuristics
> trigger.

Whilst the bootloader was the easiest place for us to replicate this
deadlock, we have also seen it with running Linux VMs.

Older Linux kernels (e.g. CentOS 5) will fall back to PIO mode on IDE
devices if they experience storage timeouts (e.g. due to heavy contention of
underlying storage from other VMs).

Hence, the IO limits deadlock can lead to running Linux VMs locking up as
well as just Windows and Linux bootloaders.

Cheers,

Richard.