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From: liu ping fan <qemulist@gmail.com>
Date: Tue, 23 Oct 2012 17:32:55 +0800
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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: Jan Kiszka <jan.kiszka@siemens.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 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:
>>>> 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
>
Yes, just ignored.  For device which support many logic in parallel,
it should use independent busy flag for each logic

Regards,
pingfan

> +    if (s->busy) {
> +        return;
>
> and
>
> +    uint64_t ret = 0;
> +
> +    if (s->busy) {
> +        return ret;
>
> is worrying me.
>
> Jan
>
> --
> Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
> Corporate Competence Center Embedded Linux