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

[Tsukasa OI <research_trasio@irq.a4lg.com>]

On 2021/12/01 17:15, Atish Patra wrote:
> On Tue, Nov 30, 2021 at 7:06 PM Tsukasa OI <research_trasio@irq.a4lg.com> wrote:
>>
>> On 2021/12/01 10:21, Atish Patra wrote:
>>> On Tue, Nov 30, 2021 at 8:13 AM Jessica Clarke <jrtc27@jrtc27.com> wrote:
>>>>
>>>> On 30 Nov 2021, at 15:01, Philipp Tomsich <philipp.tomsich@vrull.eu> 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.
> 
> The device tree need to carry a very long "riscv,isa" string. The
> parser need to parse
> that string in memory as well.
> 
>>
>> 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>
>>
> 
> It's on my todo list to review the series. I think we can work
> together to propose a better framework for riscv isa extensions.

Thanks.  I will submit RFC PATCH v3 today so that we can start a healthy
discussion.  I apologize that I missed so many points and there's a lot
things to learn.

As far as I know, if we make new DT nodes for separate extensions, we have
to (at least) synchronize the implementation with Spike.  This simulator
accepts ISA string through `--isa' option and (by default) puts entire ISA
string into the device tree as "riscv,isa" (after expansion
"G" -> "IMAFD").

Of course, it includes "Svpbmt", in which we are discussing.

    spike --isa=rv64g_zba_zbb_zbc_zbs_svinval_svnapot_svpbmt ...

I am just wondering whether breaking this behavior would worth it.

IMHO, we could create new DT nodes **and** in addition, we can possibly
use "riscv,isa" as a fallback.
I'm not sure that this would work (just changing Spike might be better)
but I ...think... it's worth discussing it.

> 
>> (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?
>>>>
> 
> Sorry. I missed this question earlier. See my answer below.
> 
>>>
>>> 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
> 
> Irrespective of the method chosen to parse the device tree in kernel,
> we need to provide the extension information to the userspace.
> 
> This is what I have in mind. An individual row with comma separated
> extension names for each type of extensions (Ss, Sv, Sh)
> after the base extension (rv64imafdc) in /proc/cpuinfo output. I am
> open to other ideas as well.
> 
> isa   rv64imafdc
> isa-ext-Sv Svpbmt
> isa-ext-Ss Sscofpmf
> isa-ext-Sh <hypervisor related extensions>
> isa-ext-Z   Zicbom
> 
> We can even explicitly name the extensions after isa-ext. However, it
> may be necessary and too long.
> 
> I guess you prefer to directly print the entire "riscv,isa" string in
> "isa" row in /proc/cpuinfo output.
> It is probably okay with the current number of extensions available
> today. However, it will become so long string
> in the future that it has to be broken into multiple lines.
> 
>> 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@jrtc27.com> wrote:
>>>>>>
>>>>>> On 30 Nov 2021, at 13:27, Heiko Stübner <heiko@sntech.de> wrote:
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> Am Dienstag, 30. November 2021, 14:17:41 CET schrieb Jessica Clarke:
>>>>>>>> On 30 Nov 2021, at 12:07, Heiko Stübner <heiko@sntech.de> 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@redhat.com wrote:
>>>>>>>>>>>> From: Wei Fu <wefu@redhat.com>
>>>>>>>>>>>>
>>>>>>>>>>>> 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@redhat.com>
>>>>>>>>>>>> Co-developed-by: Guo Ren <guoren@kernel.org>
>>>>>>>>>>>> Signed-off-by: Guo Ren <guoren@kernel.org>
>>>>>>>>>>>> Cc: Anup Patel <anup@brainfault.org>
>>>>>>>>>>>> Cc: Palmer Dabbelt <palmer@dabbelt.com>
>>>>>>>>>>>> Cc: Rob Herring <robh+dt@kernel.org>
>>>>>>>>>>>> ---
>>>>>>>>>>>> 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@mail.gmail.com/
>>>>>>>>>
>>>>>>>>> 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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>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>>
> 
> 
> 

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