Re: [PATCH v5 23/25] tests: add plugin asserting correctness of discon event's to_pc

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

On 5/20/25 2:09 PM, Pierrick Bouvier wrote:
> On 5/20/25 1:44 PM, Julian Ganz wrote:
>> Hi Pierrick,
>>
>> May 20, 2025 at 10:01 PM, Pierrick Bouvier wrote:
>>> Regarding the issue with the same tb being mapped at different virtual addresses, I'm ok with the current solution of comparing only page bits.
>>>
>>> That said, a better solution could be to compare physical addresses when a discon is detected (on plugin side), and confirm it's really a discontinuity or just a different mapping. With this approach, it's not even needed to have a dedicated option, and there would be no false positive in the plugin. It's just a suggestion though.
>>
>> I actually tried to do this before resorting to the current appraoch.
>> However, there is only API for querying an instruction's or TB's
>> hardware address and none that would let me translate the virtual
>> addresses we receive in the discon callback, which we need to compare
>> against.
>>
> 
> It would be acceptable to add such a function allowing to query physical
> address for a virtual address (using cpu_get_phys_page_debug behind the
> hoods), as it's not leaking any QEMU implementation detail.
> 
> We can implement this later if you don't want to extend your series with
> this.
>

An happy coincidence, this was just posted:
https://lore.kernel.org/qemu-devel/20250521094333.4075796-6-rowanbhart@gmail.com/

>> I considered also passing the hardware address to the callback (do the
>> translation in the `plugin_vcpu_cb__discon` hook), but that turned out
>> to be not straight forward and not something we'd want to do in the
>> hook, either.
>>
> 
> Yes, in some cases, people will want virtual addresses, and sometimes
> physical ones. So passing physical ones only is too restrictive.
> 
> This plugin is a bit specific, as it's explicitely tracking all
> transitions between instructions, where a "normal" plugin will just work
> with discontinuities. That said, the use case to get physical address
> from a virtual one is a real need.
> 
>> Regards,
>> Julian
> 
> Regards,
> Pierrick