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Date: Tue, 09 Oct 2012 15:11:06 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re:
 [RFC PATCH 00/17] Support for multiple "AIO contexts""
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To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Ping Fan Liu <pingfank@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Jan Kiszka <jan.kiszka@siemens.com>

Il 09/10/2012 14:22, Avi Kivity ha scritto:
> On 10/09/2012 02:01 PM, Paolo Bonzini wrote:
>>
>>> [could we also avoid refcounting by doing the equivalent of
>>> stop_machine() during hotunplug?]
>>
>> That's quite an interesting alternative.
> 
> It's somewhat unattractive in that we know how much stop_machine is
> hated in Linux.  But maybe it makes sense as a transitional path.
> 
> Note it's not sufficient to stop vcpu threads, we also have to stop
> non-vcpu threads that may be issuing address_space_rw() or family.

Yes, we need some list of "guest workers", of which VCPUs are just one
example.

> But no, it's actually impossible.  Hotplug may be triggered from a vcpu
> thread, which clearly it can't be stopped.

Hotplug should always be asynchronous (because that's how hardware
works), so it should always be possible to delegate the actual work to a
non-VCPU thread.  Or not?

Paolo