Re: Building QEMU as a Shared Library

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 3/11/25 02:50, Saanjh Sengupta wrote:
> Hi,
> 
> I have a couple of questions:
> 
>  1.
>     When I use the libstoptrigger.so: in that case the QEMU 's emulation
>     stops after executing the defined number of instructions. Post this,
>     the whole QEMU terminates. And while using the libips.so I am
>     assuming that the QEMU doesn't execute no more than the defined
>     instructions. Please correct me if I am wrong.

That's correct for both plugins, with the additional note that libips 
does this per second only.

>  2.
>     In my case, I want the QEMU to start emulation for some time and
>     PAUSE it's emulation for some time; after it is Paused (it's virtual
>     time is also to be paused) and then let's say for after 'x' time
>     period it should resume it's virtual time.
> 

The virtual time variable in ips plugin is only related to this plugin, 
and based on how many instructions have been executed, which is 
different from what you want to achieve.

Stoptrigger might be a better fit for what you want to do, and instead 
of exiting, you want to resume emulation after N insn.
The function qemu_clock_advance_virtual_time() can only be used to move 
the time forward, and you can not stop the "virtual time" by design.

> image
> 
> 
> I have added this segment inside the update_system_time function inside 
> the ipsPlugin.c. but once the instructions reach to the defined limit 
> the virtual time does not seem to stop.
> Do you have any suggestions on that front?
> 
> 
> Regards
> Saanjh Sengupta
> ------------------------------------------------------------------------
> *From:* Pierrick Bouvier <pierrick.bouvier@linaro.org>
> *Sent:* Wednesday, March 5, 2025 5:20:38 AM
> *To:* Saanjh Sengupta <saanjhsengupta@outlook.com>; Philippe Mathieu- 
> Daudé <philmd@linaro.org>; Paolo Bonzini <pbonzini@redhat.com>; Marc- 
> André Lureau <marcandre.lureau@redhat.com>
> *Cc:* amir.gonnen@neuroblade.ai <amir.gonnen@neuroblade.ai>; qemu- 
> devel@nongnu.org <qemu-devel@nongnu.org>; Alex Bennée 
> <alex.bennee@linaro.org>
> *Subject:* Re: Building QEMU as a Shared Library
> Hi Saanjh,
> 
> depending what you are trying to achieve exactly, plugins can provide a
> solution. It's convenient and you can stay on top of QEMU upstream,
> without having to create a downstream fork.
> 
> We already have plugins for stopping after a given number of
> instructions, or slow down execution of a VM:
> 
> # stop after executing 1'000'000 instructions:
> $ ./build/qemu-system-x86_64 -plugin
> ./build/contrib/plugins/libstoptrigger,icount=1000000 -d plugin
> 
> # execute no more than 1'000'000 instructions per second:
> $ ./build/qemu-system-x86_64 -plugin
> ./build/contrib/plugins/libips.so,ips=1000000 -d plugin
> 
> You can see source code associated (./contrib/plugins/stoptrigger.c and
> ./contrib/plugins/ips.c), to implement something similar to what you
> want, but based on time.
> Would that satisfy your need?
> 
> Regards,
> Pierrick
> 
> On 3/3/25 21:53, Saanjh Sengupta wrote:
>> 
>> 
>> Hi,
>> 
>> Thank you so much for your inputs. I was able to create the .so file of 
>> QEMU.
>> 
>> Actually, what we are trying is to understand and explore possibilities 
>> of Virtual Time Control in QEMU. In short, what I mean to say is an 
>> approach via which I can tell QEMU to emulate for XYZ time when the I 
>> give a trigger and then pause the emulation by itself after the XYZ time 
>> is completed.
>> 
>> On that front itself, do you have any inputs/ideas regarding the same?
>> 
>> 
>> Regards
>> Saanjh Sengupta
>> ------------------------------------------------------------------------
>> *From:* Pierrick Bouvier <pierrick.bouvier@linaro.org>
>> *Sent:* Tuesday, February 25, 2025 6:29:44 AM
>> *To:* Philippe Mathieu-Daudé <philmd@linaro.org>; Paolo Bonzini 
>> <pbonzini@redhat.com>; Marc-André Lureau <marcandre.lureau@redhat.com>
>> *Cc:* amir.gonnen@neuroblade.ai <amir.gonnen@neuroblade.ai>; qemu- 
>> devel@nongnu.org <qemu-devel@nongnu.org>; Saanjh Sengupta 
>> <saanjhsengupta@outlook.com>
>> *Subject:* Re: Building QEMU as a Shared Library
>> Hi Saanjh,
>> 
>> here is a minimal patch that builds one shared library per target (arch,
>> mode) where arch is cpu arch, and mode is system or user, and launch
>> system-aarch64 through a simple driver:
>> 
>> https://github.com/pbo-linaro/qemu/commit/ <https://github.com/pbo- 
> linaro/qemu/commit/>
>> fbb39cc64f77d4bf1e5e50795c75b62735bf5c5f <https://github.com/pbo-linaro/
>> qemu/commit/fbb39cc64f77d4bf1e5e50795c75b62735bf5c5f>
>> 
>> With this, it could be possible to create a driver that can execute any
>> existing target. It's a sort of single binary for QEMU, but shared
>> objects are mandatory, and duplicates all the QEMU state. So there is no
>> real benefit compared to having different processes.
>> 
>> In more, to be able to do concurrent emulations, there are much more
>> problems to be solved. QEMU state is correctly kept per target, but all
>> other libraries states are shared. There are various issues if you
>> launch two emulations at the same time in two threads:
>> - glib global context
>> - qemu calls exit in many places, which stops the whole process
>> - probably other things I didn't explore
>> 
>> At this point, even though qemu targets can be built as shared objects,
>> I would recommend to use different processes, and implement some form on
>> IPC to synchronize all this.
>> Another possibility is to try to build machines without using the
>> existing main, but I'm not sure it's worth all the hassle.
>> 
>> What are you trying to achieve?
>> 
>> Regards,
>> Pierrick
>> 
>> On 2/24/25 01:10, Philippe Mathieu-Daudé wrote:
>>> Cc'ing our meson experts
>>> 
>>> On 22/2/25 14:36, Saanjh Sengupta wrote:
>>>> Hi,
>>>>
>>>> I referred to your mailing chains on suggesting QEMU to be built as a
>>>> shared library.
>>>>
>>>> *Change meson.build to build QEMU as a shared library (with PIC enabled
>>>> for static libraries)*
>>>> *
>>>> *
>>>> Could you please suggest what exactly has to be enabled in the meson.build?
>>>>
>>>> I am confused on that front.
>>>>
>>>> Regards
>>>> Saanjh Sengupta
>>> 
>> 
>