[PATCH 5/7] RISC-V: fix auipc-jalr addresses in patched alternatives

[Heiko Stübner]

Am Montag, 21. November 2022, 23:17:11 CET schrieb Heiko Stübner:
> Am Montag, 21. November 2022, 22:31:36 CET schrieb Lad, Prabhakar:
> > Some more information,
> > 
> > - If I drop the riscv_alternative_fix_auipc_jalr() call after
> > patch_text_nosync() and then print the alt->old_ptr instructions
> > before patching I can see the instructions as 0x13 (nop) which is
> > correct.
> > 
> > - if I call riscv_alternative_fix_auipc_jalr() call after
> > patch_text_nosync() and then print the alt->old_ptr instructions
> > before patching I dont see 0x13 (nop) consistently for old
> > instructions.
> 
> which is to be expected I guess.
> 
> alt->old_ptr points to the memory location where the live kernel code
> lives.
> 
> I.e. the code at this location is the thing the kernel actually runs.
> The code at this location then gets overwritten by the alternative
> assembly.
> 
> 
> > - If I replace the nop's in the old instructions with my assembly code
> > of rz/five cmo and then just use patch_text_nosync() I can see the
> > correct actual instruction being printed apart from jalr (is some sort
> > of offset added to it as I see last 4 bits match?) and then is
> > replaced correctly by the same alt instructions apart from the jalr
> > (log [0]).
> > 
> > - If I replace the nop's in the old instructions with my assembly code
> > of rz/five cmo and then use patch_text_nosync() and
> > riscv_alternative_fix_auipc_jalr() I can see the actual old
> > instructions differs a bit and again the jalr instruction differs too
> > in the patched code (log [1]).
> > 
> > [0] https://paste.debian.net/1261412/
> > [1] https://paste.debian.net/1261413/
> > 
> > Attached is the objump of dma-noncoherent.o for reference.
> 
> I did read that objdumps are not really conclusive when looking
> at auipc + jalr instructions, hence the printing of the actual instructions.
> 
> As either manually or with a helper like
> 
> 	https://luplab.gitlab.io/rvcodecjs/#q=0xf4c080e7
> 
> you can then decode the actual instruction and compare.
> 
> In your log the two jalr instructions decode to different offsets,
> 	jalr x1, x1, -180
> vs
> 	jalr x1, x1, -834
> 
> Can you check what the patch_offset value is in your case?
> 
> Interestingly the
> 	auipc x1, 0
> is 0 for both cases.
> 
> I'll try to build a real test-setup mimicing what you're doing
> tomorrow (european tomorrow).

also, is it possible for you to put your code on some github
or so? Sometimes looking at the actual code makes things
a lot easier :-)

Thanks
Heiko