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

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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>
This commit is contained in:
David Hildenbrand 2025-02-10 09:46:48 +01:00 • committed by Peter Xu
parent 425ce9b37b
commit 1cceedd772
4 changed files with 17 additions and 13 deletions
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5
include/exec/memattrs.h
View file

@ -44,6 +44,8 @@ typedef struct MemTxAttrs {
* (see MEMTX_ACCESS_ERROR).
*/
unsigned int memory:1;
/* Debug access that can even write to ROM. */
unsigned int debug:1;
/* Requester ID (for MSI for example) */
unsigned int requester_id:16;
@ -56,7 +58,8 @@ typedef struct MemTxAttrs {
* Bus masters which don't specify any attributes will get this
* (via the MEMTXATTRS_UNSPECIFIED constant), so that we can
* distinguish "all attributes deliberately clear" from
* "didn't specify" if necessary.
* "didn't specify" if necessary. "debug" can be set alongside
* "unspecified".
*/
bool unspecified;