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, 9 insertions, 4 deletions

diff --git a/hw/core/cpu-system.c b/hw/core/cpu-system.c
index 6aae28a..6e307c8 100644
--- a/hw/core/cpu-system.c
+++ b/hw/core/cpu-system.c
@@ -51,13 +51,18 @@ hwaddr cpu_get_phys_page_attrs_debug(CPUState *cpu, vaddr addr,
                                      MemTxAttrs *attrs)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);
+    hwaddr paddr;
 
     if (cc->sysemu_ops->get_phys_page_attrs_debug) {
-        return cc->sysemu_ops->get_phys_page_attrs_debug(cpu, addr, attrs);
+        paddr = cc->sysemu_ops->get_phys_page_attrs_debug(cpu, addr, attrs);
+    } else {
+        /* Fallback for CPUs which don't implement the _attrs_ hook */
+        *attrs = MEMTXATTRS_UNSPECIFIED;
+        paddr = cc->sysemu_ops->get_phys_page_debug(cpu, addr);
     }
-    /* Fallback for CPUs which don't implement the _attrs_ hook */
-    *attrs = MEMTXATTRS_UNSPECIFIED;
-    return cc->sysemu_ops->get_phys_page_debug(cpu, addr);
+    /* Indicate that this is a debug access. */
+    attrs->debug = 1;
+    return paddr;
 }
 
 hwaddr cpu_get_phys_page_debug(CPUState *cpu, vaddr addr)