Re: [Qemu-devel] [patch v4 13/16] e1000: add busy flag to anti broken device state

[Avi Kivity]

On 10/25/2012 11:00 AM, Peter Maydell wrote:
> On 23 October 2012 10:37, Avi Kivity <address@hidden> wrote:
>> On 10/23/2012 11:32 AM, liu ping fan wrote:
>>> On Tue, Oct 23, 2012 at 5:07 PM, Jan Kiszka <address@hidden> wrote:
>>>> On 2012-10-23 07:52, liu ping fan wrote:
>>>>> On Mon, Oct 22, 2012 at 6:40 PM, Avi Kivity <address@hidden> 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.

That's why we have a per-device lock.  The busy flag breaks that model
by discarding accesses that occur in parallel.


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