physmem: teach cpu_memory_rw_debug() to write to more memory regions

Right now, we only allow for writing to memory regions that allow direct
access using memcpy etc; all other writes are simply ignored. This
implies that debugging guests will not work as expected when writing
to MMIO device regions.

Let's extend cpu_memory_rw_debug() to write to more memory regions,
including MMIO device regions. Reshuffle the condition in
memory_access_is_direct() to make it easier to read and add a comment.

While this change implies that debug access can now also write to MMIO
devices, we now are also permit ELF image loads and similar users of
cpu_memory_rw_debug() to write to MMIO devices; currently we ignore
these writes.

Peter assumes [1] that there's probably a class of guest images, which
will start writing junk (likely zeroes) into device model registers; we
previously would silently ignore any such bogus ELF sections. Likely
these images are of questionable correctness and this can be ignored. If
ever a problem, we could make these cases use address_space_write_rom()
instead, which is left unchanged for now.

This patch is based on previous work by Stefan Zabka.

[1] https://lore.kernel.org/all/CAFEAcA_2CEJKFyjvbwmpt=on=GgMVamQ5hiiVt+zUr6AY3X=Xg@mail.gmail.com/

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/213
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250210084648.33798-8-david@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/exec/memory.h b/include/exec/memory.h
index b18ecf933e..78c4e0aec8 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -3018,7 +3018,8 @@ static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write,
     if (!memory_region_supports_direct_access(mr)) {
         return false;
     }
-    if (is_write) {
+    /* Debug access can write to ROM. */
+    if (is_write && !attrs.debug) {
         return !mr->readonly && !mr->rom_device;
     }
     return true;