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

[Avi Kivity <avi@redhat.com>]

On 09/19/2012 07:40 AM, Peter Crosthwaite wrote:
> On Wed, Sep 19, 2012 at 2:32 PM, Edgar E. Iglesias
> <edgar.iglesias@gmail.com> 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.

You could schedule a bottom half and do the accesses from there.  Solves
nothing though.

> 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.

Did you mean loop, or recursion into the memory region that initiates DMA?

As this problem already exists I don't think new device models need
block on it.

Maybe a simple fix is:

void my_device_write(...)
{
   if (dma_in_progress) {
       return;
   }
   ...
   s->dma_in_progress = true;
   cpu_physical_memory_rw(...)
   s->dma_in_progress = false;
}

and we could try to figure out how to move this into common code.
-- 

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