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

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Wed, Sep 19, 2012 at 03:12:12PM +0300, Avi Kivity wrote:
> On 09/19/2012 02:46 PM, Edgar E. Iglesias wrote:
> > On Wed, Sep 19, 2012 at 10:55:30AM +0300, Avi Kivity wrote:
> >> 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?
> > 
> > I think we were discussing the loop issue (I hadn't thought of recursion
> > issues before) :)
> > 
> > Jan's point of resetability was interesting.
> > 
> > Processing a finite amount of dma descriptors and scheduling bh's
> > to continously get back and continue processing should be OK no?
> > 
> 
> Yes.
> 
> > That would avoid the lockups and allow the device to be reset at
> > any time. Or am I missing something?
> > 
> 
> Not that I can see.  If real hardware can be looped, so can qemu.  I'm
> only worried about recursion and deadlocks (while real hardware can
> deadlock, we'd prefer to avoid that).

Agreed, thanks