Re: [RFC PATCH v3 11/11] tests: add plugin asserting correctness of discon event's to_pc

[Julian Ganz]

Hi Pierrick,

December 5, 2024 at 6:30 PM, "Pierrick Bouvier" wrote:
> On 12/5/24 05:10, Julian Ganz wrote:
> >  December 5, 2024 at 12:33 AM, "Pierrick Bouvier" wrote:
> > > Trap target PC mismatch
> > >  Expected: 23faf3a80
> > >  Encountered: 23faf3a84
> > > 
> > >  From what I understand, it means that the next_pc we have is incorrect.
> > > 
> >  Yes, this is indeed incorrect, and also a perfect example why this test
> >  plugin exists. There are likely other errors lurking in target specific
> >  code. Did you happen to also log interrupts? Do you remember what image
> >  you used?
> > 
> I used exactly this:
> 
> ./build/qemu-system-aarch64 -plugin ./build/tests/tcg/plugins/libdiscons.so 
> -smp 4 -M virt -d plugin -m 8G -device virtio-blk-pci,drive=root -drive 
> if=none,id=root,file=./debianaarch64.img -M virt -cpu max,pauth=off -drive 
> if=pflash,readonly=on,file=/usr/share/AAVMF/AAVMF_CODE.fd -drive 
> if=pflash,file=./AAVMF_VARS.fd
> 
> The arm64 image is a vanilla debian stable I installed.
> AAVMF* files come from qemu-efi-aarch64 debian package.

Thanks! I will have a closer look and include a fix in the next iteration.

> > Btw: this also illustrates another issue I have with from_pc: we can
> >  test the behavior for to_pc, but doing this meaningfully for from_pc
> >  via a plugin is next to impossible because the instruction it points to
> >  is not observable via an exec callback. At least not in the general
> >  case, even not if we only consider a single type of event.
> > 
> We can store the next_expected pc for each instruction (from 
> current_instruction + insn_length), and we should be able to compare that 
> with the expected from_pc.
> This is mostly what contrib/plugins/cflow.c does.
> 
> With that, we can test from_pc.

I'm not confident that this will work reliably for branch, jump and
other "interesting" instructions. But I can have a closer look at the
cflow plugin and try to figure out how that plugin handles those cases.

Regards,
Julian Ganz