Re: [PATCH v12 0/7] Add additional plugin API functions to read and write memory and registers

["Alex Bennée" <alex.bennee@linaro.org>]

Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:

> On 6/11/25 4:24 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).
>> v11 makes the cb flags functions inline and fixes a typo where cpu
>> was asserted
>> but current_cpu was actually accessed.
>> v12 removes the hypercalls plugin because the functions it tested
>> are also
>> tested by the patcher plugin, making it redundant. We'll circle back on a
>> hypercalls API in the future as a part of the plugin API, not as a plugin
>> itself.
>> Rowan Hart (1):
>>    plugins: Add enforcement of QEMU_PLUGIN_CB flags in register R/W
>>      callbacks
>> novafacing (6):
>>    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: 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                     |  15 ++
>>   include/qemu/qemu-plugin.h                | 176 ++++++++++++++--
>>   plugins/api.c                             | 135 +++++++++++-
>>   plugins/core.c                            |  33 +++
>>   tests/tcg/Makefile.target                 |   1 +
>>   tests/tcg/plugins/meson.build             |   2 +-
>>   tests/tcg/plugins/patch.c                 | 241 ++++++++++++++++++++++
>>   tests/tcg/x86_64/Makefile.softmmu-target  |  32 ++-
>>   tests/tcg/x86_64/system/patch-target.c    |  27 +++
>>   tests/tcg/x86_64/system/validate-patch.py |  39 ++++
>>   14 files changed, 725 insertions(+), 23 deletions(-)
>>   create mode 100644 tests/tcg/plugins/patch.c
>>   create mode 100644 tests/tcg/x86_64/system/patch-target.c
>>   create mode 100755 tests/tcg/x86_64/system/validate-patch.py
>> 
>
> @Alex,
> series looks good to me now.
>
> Would you like to add comments, or is it good for you also?

I'll do a pass through next week but I think we are in good shape.

>
> Thanks,
> Pierrick

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro