Re: [Qemu-devel] [PATCH V3 10/11] vcpu: introduce lockmap

[Peter Crosthwaite]

On Wed, Sep 19, 2012 at 2:32 PM, Edgar E. Iglesias
<address@hidden> wrote:
> On Wed, Sep 19, 2012 at 02:25:48PM +1000, Peter Crosthwaite wrote:
>> Ping for PMM,
>>
>> This is the root case of your block on the SDHCI series - this is a
>> discussion on resolution to bogus infinite looping DMA. For current
>> participants in this discussion, heres our thread on the same topic
>> over in SD land:
>>
>> http://lists.gnu.org/archive/html/qemu-devel/2012-08/msg01017.html
>>
>> With the findings here and acknowledgement that this is a general
>> problem, we either need to declare this issue of scope for SDHCI, or
>> work with these guys (in the immediate future) on the DMA infinite
>> loop problem flagged here. I dont mind if SDHCI+ADMA is the guinea pig
>> here, but can we get a decisive plan for going forward with this issue
>> if it is going to continue to block SDHCI.
>>
>> Thanks to Igor for identifying the overlap between these discussions.
>
> Hi,
>
> A couple of dumb questions.
>
> What is the reason for the blocker? that possible guest dos is worse
> than no functionality at all?
>
> Can't you put the DMA/IO processing into the background?

I dont know a way do do asynchronous DMA unless I am missing something
here. So what happens is we have a device that walks a SG list
synchronously while holding all the locks and stuff being discussed
here. If that SG list infinite loops then crash.

>
> what's the diff between setup of bad descriptors and  writing a
> while (1) loop in the guest CPU?
>

While(1) in guest doesnt freeze all of QEMU (monitor still works and
stuff), wheras the DMA ops going in circles does, as locks are taken
by an infinite loop.

Regards,
peter

> Cheers,
> E