[PATCH v2 1/3] memory: Track whether a Device is engaged in IO

[Alexander Bulekov]

Add a flag to the DeviceState, when a device is engaged in PIO/MMIO/DMA.
This flag should be set/checked prior to calling a device's MemoryRegion
handlers, and set when device code initiates DMA.  The purpose of this
flag is to prevent DMA reentrancy issues. E.g.:
sdhci pio -> dma write -> sdhci mmio
nvme bh -> dma write -> nvme mmio

These issues have led to problems such as stack-exhaustion and
use-after-frees.

Assumptions:
 * Devices do not interact with their own PIO/MMIO memory-regions using
   DMA.

 * There is now way for there to be multiple simultaneous accesses to a
   device's PIO/MMIO memory-regions, or for multiple threads to perform
   DMA accesses simultaneously on behalf of a single device.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 include/hw/qdev-core.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 92c3d65208..6474dc51fa 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -193,6 +193,9 @@ struct DeviceState {
     int instance_id_alias;
     int alias_required_for_version;
     ResettableState reset;
+
+    /* Is the device currently in mmio/pio/dma? Used to prevent re-entrancy */
+    int engaged_in_io;
 };
 
 struct DeviceListener {
-- 
2.33.0