Re: [PATCH V4 1/2] dt-bindings: riscv: add MMU Standard Extensions support for Svpbmt

[Tsukasa OI]

On 2021/12/01 10:21, Atish Patra wrote:
> On Tue, Nov 30, 2021 at 8:13 AM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>>
>> On 30 Nov 2021, at 15:01, Philipp Tomsich <philipp.tomsich@xxxxxxxx> wrote:
>>>
>>> We did touch on this in our coordination call a few weeks ago: the
>>> grouping under mmu and the bool-entries were chosen because of their
>>> similarity to other extensions (i.e. for Zb[abcs] there could/should
>>> be a bool-entry under each cpu-node — for some Zv* entries a subnode
>>> might be needed with further parameters).
>>>
>>> The string-based approach (as in the originally proposed "mmu-type=")
>>> would like not scale with the proliferation of small & modular
>>> extensions.
>>
>> I don’t see why the Sv* extensions need to be under an mmu node then,
>> unless the intent is that every extension be grouped under a sub-node
>> (which doesn’t seem viable due to extensions like Zbk*, unless you
>> group by Ss, Sv and Z)?
>>
> 
> It shouldn't be. All the ISA extensions (i.e. standard, supervisor & hypervisor)
> with prefix S,Z,H should be kept separate in a separate node for easy
> parsing.

"Easy parsing" is not quite convincing.

There's a reason other than that I made RFC PATCH to parse
multi-letter extensions:

v1: <http://lists.infradead.org/pipermail/linux-riscv/2021-November/010252.html>
v2: <http://lists.infradead.org/pipermail/linux-riscv/2021-November/010350.html>

(note: those patches will break RISC-V KVM because of possible ISA
       Manual inconsistency and discussion/resolution needed)

(...continued below...)

> 
> "riscv,isa" dt property will not scale at all. Just look at the few
> extensions that were ratified this year
> and Linux kernel needs to support them.
> 
> "Sscofpmf", "Svpbmt", "Zicbom"
> 
>> Also, what is going to happen to the current riscv,isa? Will that
>> continue to exist and duplicate the info, or will kernels be required
>> to reconstruct the string themselves if they want to display it to
>> users?
>>
> 
> This is my personal preference:
> riscv,isa will continue to base Standard ISA extensions that have
> single letter extensions.
> 
> This new DT node will encode all the non-single letter extensions.
> I am not sure if it should include some provisions for custom
> extensions starting with X because
> that will be platform specific.
> 
> Again, this is just my personal preference. I will try to send a patch
> soon so that we can initiate a broader
> discussion of the scheme and agree/disagree on something.

For supervisor-only extensions like "Svpbmt", new DT node would be a
reasonable solution (and I would not directly object about that node).

However, there's many multi-letter extensions that are useful for
user mode.  Because "riscv,isa" is exposed via sysfs and procfs
(/proc/cpuinfo), it can be really helpful to have multi-letter
extensions.  Also, current version of Spike, a RISC-V ISA Simulator
puts all multi-letter extensions in "riscv,isa" and I thought this is
intended.

My preference:
(1) Allow having multi-letter extensions and versions in "riscv,isa"
(2) Adding new DT node for supervisor-related extensions would be
    reasonable (but I don't strongly agree/disagree).

Thanks,
Tsukasa

> 
> 
> 
>> As a FreeBSD developer I’m obviously not a part of many of these
>> discussions, but what the Linux community imposes as the device tree
>> bindings has a real impact on us.
>>
>> Jess
>>
>>> On Tue, 30 Nov 2021 at 14:59, Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>>>>
>>>> On 30 Nov 2021, at 13:27, Heiko Stübner <heiko@xxxxxxxxx> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> Am Dienstag, 30. November 2021, 14:17:41 CET schrieb Jessica Clarke:
>>>>>> On 30 Nov 2021, at 12:07, Heiko Stübner <heiko@xxxxxxxxx> wrote:
>>>>>>>
>>>>>>> Am Montag, 29. November 2021, 13:06:23 CET schrieb Heiko Stübner:
>>>>>>>> Am Montag, 29. November 2021, 09:54:39 CET schrieb Heinrich Schuchardt:
>>>>>>>>> On 11/29/21 02:40, wefu@xxxxxxxxxx wrote:
>>>>>>>>>> From: Wei Fu <wefu@xxxxxxxxxx>
>>>>>>>>>>
>>>>>>>>>> Previous patch has added svpbmt in arch/riscv and add "riscv,svpmbt"
>>>>>>>>>> in the DT mmu node. Update dt-bindings related property here.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Wei Fu <wefu@xxxxxxxxxx>
>>>>>>>>>> Co-developed-by: Guo Ren <guoren@xxxxxxxxxx>
>>>>>>>>>> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
>>>>>>>>>> Cc: Anup Patel <anup@xxxxxxxxxxxxxx>
>>>>>>>>>> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
>>>>>>>>>> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
>>>>>>>>>> ---
>>>>>>>>>> Documentation/devicetree/bindings/riscv/cpus.yaml | 10 ++++++++++
>>>>>>>>>> 1 file changed, 10 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml b/Documentation/devicetree/bindings/riscv/cpus.yaml
>>>>>>>>>> index aa5fb64d57eb..9ff9cbdd8a85 100644
>>>>>>>>>> --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
>>>>>>>>>> +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
>>>>>>>>>> @@ -63,6 +63,16 @@ properties:
>>>>>>>>>>      - riscv,sv48
>>>>>>>>>>      - riscv,none
>>>>>>>>>>
>>>>>>>>>> +  mmu:
>>>>>>>>>
>>>>>>>>> Shouldn't we keep the items be in alphabetic order, i.e. mmu before
>>>>>>>>> mmu-type?
>>>>>>>>>
>>>>>>>>>> +    description:
>>>>>>>>>> +      Describes the CPU's MMU Standard Extensions support.
>>>>>>>>>> +      These values originate from the RISC-V Privileged
>>>>>>>>>> +      Specification document, available from
>>>>>>>>>> +      https://riscv.org/specifications/
>>>>>>>>>> +    $ref: '/schemas/types.yaml#/definitions/string'
>>>>>>>>>> +    enum:
>>>>>>>>>> +      - riscv,svpmbt
>>>>>>>>>
>>>>>>>>> The privileged specification has multiple MMU related extensions:
>>>>>>>>> Svnapot, Svpbmt, Svinval. Shall they all be modeled in this enum?
>>>>>>>>
>>>>>>>> I remember in some earlier version some way back there was the
>>>>>>>> suggestion of using a sub-node instead and then adding boolean
>>>>>>>> properties for the supported extensions.
>>>>>>>>
>>>>>>>> Aka something like
>>>>>>>>   mmu {
>>>>>>>>           riscv,svpbmt;
>>>>>>>>   };
>>>>>>>
>>>>>>> For the record, I'm talking about the mail from september
>>>>>>> https://lore.kernel.org/linux-riscv/CAAeLtUChjjzG+P8yg45GLZMJy5UR2K5RRBoLFVZhtOaZ5pPtEA@xxxxxxxxxxxxxx/
>>>>>>>
>>>>>>> So having a sub-node would make adding future extensions
>>>>>>> way nicer.
>>>>>>
>>>>>> Svpbmt is just an ISA extension, and should be treated like any other.
>>>>>> Let’s not invent two different ways of representing that in the device
>>>>>> tree.
>>>>>
>>>>> Heinrich asked how the other extensions should be handled
>>>>> (Svnapot, Svpbmt, Svinval), so what do you suggest to do with these?
>>>>
>>>> Whatever is done for Zb[abcs], Zk*, Zv*, Zicbo*, etc. There may not be
>>>> a concrete plan for that yet, but that means you should speak with the
>>>> people involved with such extensions and come up with something
>>>> appropriate together.
>>>>
>>>> Jess
>>>>
>>
>>
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> 
> 
> 
> --
> Regards,
> Atish
> 
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