Re: [PATCH v10 0/8] Add additional plugin API functions to read and write memory and registers

[Rowan Hart <rowanbhart@gmail.com>]

I forgot one additional update here that was necessary because the 
plugin_cb_flags getter/setters are now on the hot path and need 
inlining, and left a typo, both of which I fixed in v11 
(https://lore.kernel.org/qemu-devel/20250609193841.348076-1-rowanbhart@gmail.com/T/#t). 


On 6/9/25 12:23 PM, Rowan Hart wrote:
> This patch series adds several new API functions focused on enabling use
> cases around reading and writing guest memory from QEMU plugins. To support
> these new APIs, some utility functionality around retrieving information about
> address spaces is added as well.
>
> The new qemu_plugin_write_register utilizes gdb_write_register, which is now
> declared in gdbstub.h for this purpose instead of being static.
>
> qemu_plugin_write_memory_vaddr utilizes cpu_memory_rw_debug much the same as
> the existing read_memory_vaddr function does.
>
> The read and write_hwaddr functions are the most different. These functions
> use address_space_rw, which works well in most cases. There is an important
> caveat that for writes, the page being written will be set dirty by the
> write operation. This dirty setting requires locking the page range,
> which can contend with an already held lock in page_collection_lock
> when called in a tb translate callback with a write to the instruction
> memory in the tb. The doc comments warn against doing this, and it's unlikely
> anyone would want to do this.
>
> I've also added two test plugins: one that implements a simple hypercall
> interface that guest code can use to communicate with the plugin in a
> structured way with a test to ensure that this hypercall works and writing
> virtual memory works. And one that implements a simple patch utility to patch
> memory at runtime. The test for the second plugin ensures the patch applies
> successfully to instruction memory, and can use both hw and vaddr methods.
>
> For v3, I've had a few comments from the last submission that I've addressed,
> and some that I haven't for one reason or another:
>
> - Enforce QEMU_PLUGIN_CB_ flags in register read/write operations: done!
> - Fix my commit messages and add long messages describing commits: done!
> - Un-expose AS internals: done! Functions operate on current vCPU, current AS.
> - Clean up use of current_cpu: done!
> - Make functions take a vcpu_idx: not done. May revisit but it allows footguns.
>    Even for translation, seems best to not do this now. We can easily add _vcpu
>    versions of these functions in the future if we change our minds!
>
> For v5, I've just updated the enforcement of the QEMU_PLUGIN_CB_ flags to just
> use immediate stores, which simplifies the implementation quite a lot and
> should be more efficient too. Thanks Pierrick for the suggestion!
>
> v6 is a formatting pass, I left some whitespace that needed removal, some
> license text was wrong, and so forth.
>
> v8 reverts a mistake I made extending the size of arrays of TCGHelperInfo
> structs, as I misunderstood their sizes. It preserves adding an explicit
> zero as the last entry for clarity, however.
>
> v9 fixes qemu_plugin_read_register to return -1 on parameter or flag state
> error instead of 0.
>
> In v10, I relaxed the restriction on when the register r/w functions can be
> called, allowing all them to be used from any callback where the CPU is not
> currently executing, with additional notes in the documentation for exceptions
> (atexit and flush, which do not operate on a specific CPU and in which
> current_cpu is not set).
>
> Rowan Hart (1):
>    plugins: Add enforcement of QEMU_PLUGIN_CB flags in register R/W
>      callbacks
>
> novafacing (7):
>    gdbstub: Expose gdb_write_register function to consumers of gdbstub
>    plugins: Add register write API
>    plugins: Add memory virtual address write API
>    plugins: Add memory hardware address read/write API
>    plugins: Add patcher plugin and test
>    plugins: Add hypercalls plugin and test
>    plugins: Update plugin version and add notes
>
>   accel/tcg/plugin-gen.c                        |  30 +
>   gdbstub/gdbstub.c                             |   2 +-
>   include/exec/gdbstub.h                        |  14 +
>   include/hw/core/cpu.h                         |   1 +
>   include/qemu/plugin.h                         |   6 +
>   include/qemu/qemu-plugin.h                    | 176 +++++-
>   plugins/api.c                                 | 135 ++++-
>   plugins/core.c                                |  45 ++
>   tests/tcg/Makefile.target                     |   2 +
>   tests/tcg/plugins/hypercalls.c                | 547 ++++++++++++++++++
>   tests/tcg/plugins/meson.build                 |   2 +-
>   tests/tcg/plugins/patch.c                     | 297 ++++++++++
>   tests/tcg/x86_64/Makefile.softmmu-target      |  36 +-
>   tests/tcg/x86_64/system/hypercalls-target.c   |  40 ++
>   tests/tcg/x86_64/system/patch-target.c        |  27 +
>   .../tcg/x86_64/system/validate-hypercalls.py  |  40 ++
>   tests/tcg/x86_64/system/validate-patch.py     |  39 ++
>   17 files changed, 1416 insertions(+), 23 deletions(-)
>   create mode 100644 tests/tcg/plugins/hypercalls.c
>   create mode 100644 tests/tcg/plugins/patch.c
>   create mode 100644 tests/tcg/x86_64/system/hypercalls-target.c
>   create mode 100644 tests/tcg/x86_64/system/patch-target.c
>   create mode 100755 tests/tcg/x86_64/system/validate-hypercalls.py
>   create mode 100755 tests/tcg/x86_64/system/validate-patch.py
>