linux-kernel - Re: [PATCH] riscv: Don't use va_pa

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Date:   Mon, 25 Oct 2021 04:00:12 +0300
From:   Nick Kossifidis <mick@....forth.gr>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     mick@....forth.gr, alexandre.ghiti@...onical.com,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Don't use va_pa_offset on kdump

Στις 2021-10-23 23:14, Palmer Dabbelt έγραψε:
> On Sat, 09 Oct 2021 06:18:48 PDT (-0700), mick@....forth.gr 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