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Date: Tue, 23 Oct 2012 11:07:11 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
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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: liu ping fan <qemulist@gmail.com>
Cc: Liu Ping Fan <pingfank@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, Marcelo Tosatti <mtosatti@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Avi Kivity <avi@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Paolo Bonzini <pbonzini@redhat.com>

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:
>>> The broken device state is caused by releasing local lock before acquiring
>>> big lock. To fix this issue, we have two choice:
>>>   1.use busy flag to protect the state
>>>     The drawback is that we will introduce independent busy flag for each
>>>     independent device's logic unit.
>>>   2.reload the device's state
>>>     The drawback is if the call chain is too deep, the action to reload will
>>>     touch each layer. Also the reloading means to recaculate the intermediate
>>>     result based on device's regs.
>>>
>>> This patch adopt the solution 1 to fix the issue.
>>
>> Doesn't the nested mmio patch detect this?
>>
> 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

+    if (s->busy) {
+        return;

and

+    uint64_t ret = 0;
+
+    if (s->busy) {
+        return ret;

is worrying me.

Jan

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