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 <alx...@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?
>
> /* Define this in the memory API */
> typedef struct {
> bool engaged_in_io;
> } MemReentrancyGuard;
>
> /* Embed MemReentrancyGuard in DeviceState */
> ...
>
> /* Require it in dma_*() APIs */
> static inline MemTxResult dma_memory_rw(AddressSpace *as, dma_addr_t addr,
> void *buf, dma_addr_t len,
> DMADirection dir, MemTxAttrs attrs,
> MemReentrancyGuard *guard);
>
> /* Call dma_*() APIs like this... */
> 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, &dev->qdev.reentrancy_guard);
> }
>
Taking a stab at this. Here is the list of DMA APIs that appear to need
changes:
dma_memory_valid (1 usage)
dma_memory_rw (~5 uses)
dma_memory_read (~92 uses)
dma_memory_write (~71 uses)
dma_memory_set (~4 uses)
dma_memory_map (~18 uses)
dma_memory_unmap (~21 uses)
{ld,st}_{le,be}_{uw,l,q}_dma (~10 uses)
ldub_dma (does not appear to be used anywhere)
stb_dma (1 usage)
dma_buf_read (~18 uses)
dma_buf_write (~7 uses)
These appear to be internal to the DMA API and probably don't need to be
changed:
dma_memory_read_relaxed (does not appear to be used anywhere)
dma_memory_write_relaxed (does not appear to be used anywhere)
dma_memory_rw_relaxed
I don't think the sglist APIs need to be changed since we can get
DeviceState from the QEMUSGList.
Does this look more-or-less right?
-Alex
