Re: [PATCH v2] memory: prevent dma-reentracy issues

[Stefan Hajnoczi]

On Wed, 13 Jul 2022 at 16:51, Alexander Bulekov <alxndr@bu.edu> wrote:
>
> On 220712 1034, Stefan Hajnoczi wrote:
> > On Tue, Jun 21, 2022 at 11:53:06AM -0400, Alexander Bulekov wrote:
> > > On 220621 1630, Peter Maydell wrote:
> > > > On Thu, 9 Jun 2022 at 14:59, Alexander Bulekov <alxndr@bu.edu> wrote:
> > > > > diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
> > > > > index 44dacfa224..ab1ad0f7a8 100644
> > > > > --- a/include/hw/pci/pci.h
> > > > > +++ b/include/hw/pci/pci.h
> > > > > @@ -834,8 +834,17 @@ static inline MemTxResult pci_dma_rw(PCIDevice 
> > > > > *dev, dma_addr_t addr,
> > > > >                                       void *buf, dma_addr_t len,
> > > > >                                       DMADirection dir, MemTxAttrs 
> > > > > attrs)
> > > > >  {
> > > > > -    return dma_memory_rw(pci_get_address_space(dev), addr, buf, len,
> > > > > -                         dir, attrs);
> > > > > +    bool prior_engaged_state;
> > > > > +    MemTxResult result;
> > > > > +
> > > > > +    prior_engaged_state = dev->qdev.engaged_in_io;
> > > > > +
> > > > > +    dev->qdev.engaged_in_io = true;
> > > > > +    result = dma_memory_rw(pci_get_address_space(dev), addr, buf, 
> > > > > len,
> > > > > +                           dir, attrs);
> > > > > +    dev->qdev.engaged_in_io = prior_engaged_state;
> > > > > +
> > > > > +    return result;
> > > >
> > > > Why do we need to do something in this pci-specific function ?
> > > > I was expecting this to only need changes at the generic-to-all-devices
> > > > level.
> > >
> > > Both of these handle the BH->DMA->MMIO case. Unlike MMIO, I don't think
> > > there is any neat way to set the engaged_in_io flag as we enter a BH. So
> > > instead, we try to set it when a device initiates DMA.
> > >
> > > The pci function lets us do that since we get a glimpse of the dev/qdev
> > > (unlike the dma_memory_...  functions).
> > ...
> > > > > @@ -302,6 +310,10 @@ static MemTxResult dma_buf_rw(void *buf, 
> > > > > dma_addr_t len, dma_addr_t *residual,
> > > > >          xresidual -= xfer;
> > > > >      }
> > > > >
> > > > > +    if (dev) {
> > > > > +        dev->engaged_in_io = prior_engaged_state;
> > > > > +    }
> > > >
> > > > Not all DMA goes through dma_buf_rw() -- why does it need changes?
> > >
> > > This one has the same goal, but accesses the qdev through sg, instead of
> > > PCI.
> >
> > Should dma_*() APIs take a reentrancy guard argument so that all DMA
> > accesses are systematically covered?
>
> That seems like it would be the best option, though it carries the cost
> of needing to tweak a lot of code in hw/. Maybe some refactoring tool
> could help with this.

Coccinelle is good at adding/removing arguments, but it doesn't know
how to get the DeviceState from, say, a PCIDevice. It may be possible
to cover the majority of cases automatically by writing a cocci
pattern that applies when the containing function takes a PCIDevice*
argument, for example. Some manual work would still be necessary.

Stefan