Re: [PATCH] riscv: Don't use va_pa_offset on kdump

[Nick Kossifidis]

Στις 2021-10-23 23:14, Palmer Dabbelt έγραψε:
> On Sat, 09 Oct 2021 06:18:48 PDT (-0700), mick@xxxxxxxxxxxx wrote:
> > Στις 2021-10-06 14:13, Alexandre Ghiti έγραψε:
> > > > +
> > > > +       /* This will trigger a jump to CSR_STVEC anyway */
> > > >         jalr    zero, a2, 0
> > >
> > > The last jump to a2 can be removed since the fault will be triggered
> > > before even reaching this instruction.
> >
> > Just switching SATP to zero doesn't generate a trap unless mstatus.TVM
> > is set (for visualization purposes). The hart will try and execute the
> > next instruction but it's not clear in the spec what happens in case 
> > the
> > code is cached, I don't want to rely solely on STVEC. I prefer having
> > this instruction there, note that some earlier QEMU versions also had
> > this behavior (the original kdump patch didn't set STVEC and it worked
> > fine after setting SATP to zero).
>
> IIRC this came down to some very specific wording in the spec.
> Something along the lines of the 0 in SATP meaning "no translation",
> SFENCE.VMA ordering translations, and the general "if the spec doesn't
> mention it then it has to work" logic.  I thought I opened a spec
> issue about this for clarification, but I can't find it.

I guess you mean this one:
https://github.com/riscv/riscv-isa-manual/issues/538

I couldn't find anything though regarding cached code, it's not that 
there's going to be a load after setting satp to 0 if the code has been 
cached, so even if the translation is cached we don't have a guarantee 
that the next instruction will result a trap.

> That said, I'm perfectly fine taking the safe approach here as it's
> not like the performance matters here.  Warrants a comment, though.

ACK

> I don't have a v2 in my inbox, did I miss something?  Also, if it's
> just the tags then it's generally not necessary to re-send something.
> The comment does, though.
>
> LMK if you want me to deal with this, or if there's going to be a v2.
>
> Thanks!

I'll send a v2 with the tags and the comment.

Regards,
Nick