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 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.

> 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