linux-kernel - Re: [PATCH v2 05/19] riscv: add ISA extension parsing for vector crypto extensions

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Date:   Thu, 19 Oct 2023 11:35:59 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     Evan Green <evan@...osinc.com>
Cc:     Jerry Shih <jerry.shih@...ive.com>,
        linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Jonathan Corbet <corbet@....net>,
        Andrew Jones <ajones@...tanamicro.com>,
        Conor Dooley <conor@...nel.org>,
        Samuel Ortiz <sameo@...osinc.com>
Subject: Re: [PATCH v2 05/19] riscv: add ISA extension parsing for vector
 crypto extensions



On 18/10/2023 19:26, Evan Green wrote:
> On Wed, Oct 18, 2023 at 5:53 AM Clément Léger <cleger@...osinc.com> wrote:
>>
>>
>>
>> On 18/10/2023 03:45, Jerry Shih wrote:
>>> On Oct 17, 2023, at 21:14, Clément Léger <cleger@...osinc.com> wrote:
>>>> @@ -221,6 +261,22 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
>>>>      __RISCV_ISA_EXT_DATA(zkt, RISCV_ISA_EXT_ZKT),
>>>>      __RISCV_ISA_EXT_DATA(zksed, RISCV_ISA_EXT_ZKSED),
>>>>      __RISCV_ISA_EXT_DATA(zksh, RISCV_ISA_EXT_ZKSH),
>>>> +    __RISCV_ISA_EXT_DATA(zvbb, RISCV_ISA_EXT_ZVBB),
>>>> +    __RISCV_ISA_EXT_DATA(zvbc, RISCV_ISA_EXT_ZVBC),
>>>> +    __RISCV_ISA_EXT_DATA(zvkb, RISCV_ISA_EXT_ZVKB),
>>>
>>> The `Zvkb` is the subset of `Zvbb`[1]. So, the `Zvkb` should be bundled with `Zvbb`.
>>
>> Hi Jerry,
>>
>> Thanks for catching this, I think some other extensions will fall in
>> this category as well then (Zvknha/Zvknhb). I will verify that.
> 
> The bundling mechanism works well when an extension is a pure lasso
> around other extensions. We'd have to tweak that code if we wanted to
> support cases like this, where the extension is a superset of others,
> but also contains loose change not present anywhere else (and
> therefore also needs to stand as a separate bit).

For Zvbb and Zvknhb, I used the following code:

static const unsigned int riscv_zvbb_bundled_exts[] = {
	RISCV_ISA_EXT_ZVKB,
	RISCV_ISA_EXT_ZVBB
};

static const unsigned int riscv_zvknhb_bundled_exts[] = {
	RISCV_ISA_EXT_ZVKNHA,
	RISCV_ISA_EXT_ZVKNHB
};

Which correctly results in both extension (superset + base set) being
enabled when only one is set. Is there something that I'm missing ?

> 
> IMO, decomposing "pure" bundles makes sense since otherwise usermode
> would have to query multiple distinct bitmaps that meant the same
> thing (eg check the Zk bit, or maybe check the Zkn/Zkr/Zkt bits, or
> maybe check the Zbkb/Zbkc... bits, and they're all equivalent). But
> when an extension is a superset that also contains loose change, there
> really aren't two equivalent bitmasks, each bit adds something new.

Agreed but if a system only report ZVBB for instance and the user wants
ZVKB, then it is clear that ZVKB should be reported as well I guess. So
in the end, it works much like "bundle" extension, just that the bundle
is actually a "real" ISA extension by itself.

Clément

> 
> There's an argument to be made for still turning on the containing
> extensions to cover for silly ISA strings (eg ISA strings that
> advertise the superset but fail to advertise the containing
> extensions). We can decide if we want to work that hard to cover
> hypothetical broken ISA strings now, or wait until they show up.
> Personally I would wait until something broken shows up. But others
> may feel differently.
> 
> -Evan