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Date: Thu, 25 Oct 2012 10:00:47 +0100
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From: Peter Maydell <peter.maydell@linaro.org>
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Subject: Re: [Qemu-devel] [patch v4 13/16] e1000: add busy flag to anti
 broken device state
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To: Avi Kivity <avi@redhat.com>
Cc: Liu Ping Fan <pingfank@linux.vnet.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, liu ping fan <qemulist@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>, Stefan Hajnoczi <stefanha@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>

On 23 October 2012 10:37, Avi Kivity <avi@redhat.com> wrote:
> On 10/23/2012 11:32 AM, liu ping fan wrote:
>> On Tue, Oct 23, 2012 at 5:07 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>>> On 2012-10-23 07:52, liu ping fan wrote:
>>>> On Mon, Oct 22, 2012 at 6:40 PM, Avi Kivity <avi@redhat.com> wrote:
>>>>> On 10/22/2012 11:23 AM, Liu Ping Fan wrote:
>>>> It will only record and fix the issue on one thread. But guest can
>>>> touch the emulated device on muti-threads.
>>>
>>> Sorry, what does that mean? A second VCPU accessing the device will
>>> simply be ignored when it races with another VCPU? Specifically
>>>
>> Yes, just ignored.  For device which support many logic in parallel,
>> it should use independent busy flag for each logic
>
> We don't actually know that e1000 doesn't.  Why won't writing into
> different registers in parallel work?

Unless the device we're emulating supports multiple in
parallel accesses (and I bet 99.9% of the devices we model
don't) then the memory framework needs to serialise the
loads/stores. Otherwise it's just going to be excessively
hard to write a reliable device model.

-- PMM