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

[Heiko Stübner]

Am Mittwoch, 1. Dezember 2021, 09:41:48 CET schrieb atish patra:
> On Wed, Dec 1, 2021 at 12:30 AM Heiko Stübner <heiko@xxxxxxxxx> wrote:
> 
> > Am Mittwoch, 1. Dezember 2021, 09:15:18 CET schrieb Atish Patra:
> > > On Tue, Nov 30, 2021 at 7:06 PM Tsukasa OI <research_trasio@xxxxxxxxxxxx>
> > wrote:
> > > >
> > > > 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.
> > >
> > > 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.
> > >
> > > > (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.
> >
> > Aren't other architectures just using a flags [x86] or features [arm64]
> > line in cpuinfo to expose the available additional cpu features
> > as a space-separated list?
> >
> > So you could also just do something similar like
> > isa:   rv64imafdc
> > isa-ext: Svpbmt Sscofpmf foo bar
> >
> >
> A space separated list is also fine by me.
> Should we keep all the extensions as one row or split based on the type of
> extensions (Ss, Sv, Sh,)?
> 
> When I look at the flags in x86, my eyes hurt badly ;)

On arm64
Features        : fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid

or on arm32
Features        : half thumb fastmult vfp edsp thumbee neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm


> That's why I suggested splitting by type of extensions to improve
> readability.

Though I guess with that split you introduce more requirements on userspace?
Because things that parse cpuinfo (think some python library) will need to
be updated when some new extension category surfaces?



> > That would make a nice compromise between length and readability
> > by users I guess?
> >
> >
> > Heiko
> >
> > > 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@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
> > > > >>>>
> > > > >>
> > > > >>
> > > > >> _______________________________________________
> > > > >> linux-riscv mailing list
> > > > >> linux-riscv@xxxxxxxxxxxxxxxxxxx
> > > > >> http://lists.infradead.org/mailman/listinfo/linux-riscv
> > > > >
> > > > >
> > > > >
> > > > > --
> > > > > Regards,
> > > > > Atish
> > > > >
> > > > > _______________________________________________
> > > > > linux-riscv mailing list
> > > > > linux-riscv@xxxxxxxxxxxxxxxxxxx
> > > > > http://lists.infradead.org/mailman/listinfo/linux-riscv
> > > > >
> > >
> > >
> > >
> > >
> >
> >
> >
> >
> >
> 
>