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

[Igor Mitsyanko]

On 09/19/2012 11:57 AM, Jan Kiszka wrote:
> On 2012-09-19 06:40, Peter Crosthwaite wrote:
> > 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.
> We could switch to async DMA, but most DMA-capable devices aren't
> prepared for rescheduling points in the middle of their code. This will
> require quite a bit of code review.

From what I understand, we can't switch to async DMA while SD card 
model doesn't support it. And introducing this support would require to 
convert every existing SD card user in QEMU.
I can recall that such a conversion was already suggested in a mailing 
list a while ago, but no one (including me) volunteered to do it :)

> > > 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.
> That's the point. If we loop, we need at least a way to stop it by
> issuing a device or system reset. But I tend to think that detecting
> loops and failing/ignoring requests is easier implementable.
>
> Jan