[RFC PATCH] memory: Fix dma-reentrancy issues at the MMIO level

[Alexander Bulekov]

Here's my shot at fixing dma-reentracy issues. This patch adds a flag to
the DeviceState, which is set/checked when we call an accessor
associated with the device's IO MRs.

The problem, in short, as I understand it: For the vast majority of
cases, we want to prevent a device from accessing it's own PIO/MMIO
regions over DMA.

This patch/solution is based on some assumptions:
1. DMA accesses that hit mmio regions are only dangerous if they end up
interacting with memory-regions belonging to the device initiating the
DMA.
Not dangerous:  sdhci_pio->dma_write->e1000_mmio
Dangerous:      sdhci_pio->dma_write->sdhci_mmio

2. Most devices do not interact with their own PIO/MMIO memory-regions
using DMA.

3. There is no way for there to be multiple simultaneous accesses to a
device's PIO/MMIO memory-regions.

4. All devices are QOMified :-)

With this patch, I wasn't able to reproduce the issues being tracked
here, with QTest reproducers:
https://gitlab.com/qemu-project/qemu/-/issues/556

This passes the i386 qos/qtests for me and I was able to boot some linux/windows
VMs with basic devices configured, without any apparent problems.

Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Mauro Matteo Cascella <mcascell@redhat.com>
Cc: Qiuhao Li <Qiuhao.Li@outlook.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Li Qiang <liq3ea@gmail.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Laurent Vivier <lvivier@redhat.com>
Cc: Bandan Das <bsd@redhat.com>
Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Cc: Darren Kenny <darren.kenny@oracle.com>
Cc: Bin Meng <bin.meng@windriver.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 include/hw/qdev-core.h |  1 +
 softmmu/memory.c       | 15 +++++++++++++++
 softmmu/trace-events   |  1 +
 3 files changed, 17 insertions(+)

diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 20d3066595..32f7c779ab 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -193,6 +193,7 @@ struct DeviceState {
     int instance_id_alias;
     int alias_required_for_version;
     ResettableState reset;
+    int engaged_in_direct_io;
 };
 
 struct DeviceListener {
diff --git a/softmmu/memory.c b/softmmu/memory.c
index 7340e19ff5..255c3c602f 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -532,6 +532,7 @@ static MemTxResult access_with_adjusted_size(hwaddr addr,
     uint64_t access_mask;
     unsigned access_size;
     unsigned i;
+    DeviceState *dev = NULL;
     MemTxResult r = MEMTX_OK;
 
     if (!access_size_min) {
@@ -541,6 +542,17 @@ static MemTxResult access_with_adjusted_size(hwaddr addr,
         access_size_max = 4;
     }
 
+    /* Do not allow more than one simultanous access to a device's IO Regions 
*/
+    if (mr->owner &&
+        !mr->ram_device && !mr->ram && !mr->rom_device && !mr->readonly) {
+        dev = (DeviceState *) object_dynamic_cast(mr->owner, TYPE_DEVICE);
+        if (dev->engaged_in_direct_io) {
+            trace_memory_region_reentrant_io(get_cpu_index(), mr, addr, size);
+            return MEMTX_ERROR;
+        }
+        dev->engaged_in_direct_io = true;
+    }
+
     /* FIXME: support unaligned access? */
     access_size = MAX(MIN(size, access_size_max), access_size_min);
     access_mask = MAKE_64BIT_MASK(0, access_size * 8);
@@ -555,6 +567,9 @@ static MemTxResult access_with_adjusted_size(hwaddr addr,
                         access_mask, attrs);
         }
     }
+    if (dev) {
+        dev->engaged_in_direct_io = false;
+    }
     return r;
 }
 
diff --git a/softmmu/trace-events b/softmmu/trace-events
index 9c88887b3c..d7228316db 100644
--- a/softmmu/trace-events
+++ b/softmmu/trace-events
@@ -13,6 +13,7 @@ memory_region_ops_read(int cpu_index, void *mr, uint64_t 
addr, uint64_t value, u
 memory_region_ops_write(int cpu_index, void *mr, uint64_t addr, uint64_t 
value, unsigned size, const char *name) "cpu %d mr %p addr 0x%"PRIx64" value 
0x%"PRIx64" size %u name '%s'"
 memory_region_subpage_read(int cpu_index, void *mr, uint64_t offset, uint64_t 
value, unsigned size) "cpu %d mr %p offset 0x%"PRIx64" value 0x%"PRIx64" size 
%u"
 memory_region_subpage_write(int cpu_index, void *mr, uint64_t offset, uint64_t 
value, unsigned size) "cpu %d mr %p offset 0x%"PRIx64" value 0x%"PRIx64" size 
%u"
+memory_region_reentrant_io(int cpu_index, void *mr, uint64_t offset, unsigned 
size) "cpu %d mr %p offset 0x%"PRIx64" size %u"
 memory_region_ram_device_read(int cpu_index, void *mr, uint64_t addr, uint64_t 
value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" size %u"
 memory_region_ram_device_write(int cpu_index, void *mr, uint64_t addr, 
uint64_t value, unsigned size) "cpu %d mr %p addr 0x%"PRIx64" value 0x%"PRIx64" 
size %u"
 memory_region_sync_dirty(const char *mr, const char *listener, int global) "mr 
'%s' listener '%s' synced (global=%d)"
-- 
2.33.0