Re: [RFC PATCH v2 0/5] physmem: Have flaview API check bus permission from MemTxAttrs argument

[Gerd Hoffmann]

  Hi,

> I was vaguely tossing an idea around in the back of my mind
> about whether you could have a flag on devices that marked
> them as "this device is currently involved in IO", such that
> you could then just fail the last DMA (or qemu_irq_set, or
> whatever) that would complete the loop back to a device that
> was already doing IO. But that would need a lot of thinking
> through to figure out if it's feasible, and it's probably
> a lot of code change.

Quick & dirty hack trying the above.  Not much code, it is opt-in per
MemoryRegion (so less overhead for devices which already handle all DMA
in a BH), tracks state in DeviceState.  Adds a check to a rather hot
code path though.  Not tested yet (stopped investigating when I noticed
Philippe tries to fix the same thing with another approach).  Not
benchmarked.

Maybe it helps ...

take care,
  Gerd

>From 80e58a2cd2c630f0bddd9d0eaee71abb7eeb9440 Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Tue, 17 Aug 2021 07:35:37 +0200
Subject: [PATCH] allow track active mmio handlers

---
 include/exec/memory.h  |  1 +
 include/hw/qdev-core.h |  1 +
 softmmu/memory.c       | 24 ++++++++++++++++++++++--
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index c3d417d317f0..b1883d45e817 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -265,6 +265,7 @@ struct MemoryRegionOps {
          */
         bool unaligned;
     } impl;
+    bool block_reenter;
 };
 
 typedef struct MemoryRegionClass {
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index bafc311bfa1b..4cf281a81fa9 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -191,6 +191,7 @@ struct DeviceState {
     int instance_id_alias;
     int alias_required_for_version;
     ResettableState reset;
+    bool io_handler_active;
 };
 
 struct DeviceListener {
diff --git a/softmmu/memory.c b/softmmu/memory.c
index bfedaf9c4dfc..5eb5dd465dd2 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -437,7 +437,18 @@ static MemTxResult  
memory_region_read_accessor(MemoryRegion *mr,
 {
     uint64_t tmp;
 
-    tmp = mr->ops->read(mr->opaque, addr, size);
+    if (mr->ops->block_reenter) {
+        DeviceState *dev = DEVICE(mr->owner);
+        if (!dev->io_handler_active) {
+            dev->io_handler_active = true;
+            tmp = mr->ops->read(mr->opaque, addr, size);
+            dev->io_handler_active = false;
+        } else {
+            tmp = MEMTX_OK;
+        }
+    } else {
+        tmp = mr->ops->read(mr->opaque, addr, size);
+    }
     if (mr->subpage) {
         trace_memory_region_subpage_read(get_cpu_index(), mr, addr, tmp, size);
     } else if (trace_event_get_state_backends(TRACE_MEMORY_REGION_OPS_READ)) {
@@ -489,7 +500,16 @@ static MemTxResult 
memory_region_write_accessor(MemoryRegion *mr,
         trace_memory_region_ops_write(get_cpu_index(), mr, abs_addr, tmp, size,
                                       memory_region_name(mr));
     }
-    mr->ops->write(mr->opaque, addr, tmp, size);
+    if (mr->ops->block_reenter) {
+        DeviceState *dev = DEVICE(mr->owner);
+        if (!dev->io_handler_active) {
+            dev->io_handler_active = true;
+            mr->ops->write(mr->opaque, addr, tmp, size);
+            dev->io_handler_active = false;
+        }
+    } else {
+        mr->ops->write(mr->opaque, addr, tmp, size);
+    }
     return MEMTX_OK;
 }
 
-- 
2.31.1