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

[Evan Green]

On Mon, Oct 23, 2023 at 12:24 AM Clément Léger <cleger@xxxxxxxxxxxx> wrote:
>
>
>
> On 19/10/2023 18:19, Evan Green wrote:
> > On Thu, Oct 19, 2023 at 8:33 AM Conor Dooley <conor@xxxxxxxxxx> wrote:
> >>
> >> On Thu, Oct 19, 2023 at 11:35:59AM +0200, Clément Léger wrote:
> >>>
> >>>
> >>> On 18/10/2023 19:26, Evan Green wrote:
> >>>> On Wed, Oct 18, 2023 at 5:53 AM Clément Léger <cleger@xxxxxxxxxxxx> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>> On 18/10/2023 03:45, Jerry Shih wrote:
> >>>>>> On Oct 17, 2023, at 21:14, Clément Léger <cleger@xxxxxxxxxxxx> 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.
> >>
> >> I'm not really sure that those are "silly" ISA strings. People are going
> >> to do it that way because it is much easier than spelling out 5 dozen
> >> sub-components, and it is pretty inevitable that subsets will be
> >> introduced in the future for extensions we currently have.
> >>
> >> IMO, it's perfectly valid to say you have the supersets and not spell
> >> out all the subcomponents.
> >
> > Hm, ok. If ISA strings are likely to be written that way, then I agree
> > having the kernel flip on all the contained extensions is a good idea.
> > We can tweak patch 2 to support the parsing of struct
> > riscv_isa_ext_data with both .id and .bundle_size set (instead of only
> > one or the other as it is now). Looking back at that patch, it looks
> > quite doable. Alright!
>
> Hey Evan,
>
> do you have anything against using this code:
>
> static const unsigned int riscv_zvbb_bundled_exts[] = {
>         RISCV_ISA_EXT_ZVKB,
>         RISCV_ISA_EXT_ZVBB
> };
>
> ...
>
> Then declaring zvbb like that:
>
> __RISCV_ISA_EXT_BUNDLE(zvbb, riscv_zvbb_bundled_exts),
>
> I agree that it is *not* a bundled extension but it actually already
> works with Conor's code. Not sure that adding more code is needed to
> handle that case.

Ah, I had missed that Zvbb was in Zvbb's own bundle. I see now that it
works, but it also feels a bit like we're working around our own code.

An alternate way, which you can decide if you like better, would be a
new macro (something like __RISCV_ISA_EXT_DATA_BUNDLE(), but better
names welcome) that allows setting both .id and .bundle_size. Then the
else-if in match_isa_ext() could just turn into two independent ifs.
You'd have to define an "invalid" value for .id, since 0 is 'a', but
that should be straightforward. Or maybe jiggle things around a bit so
0 is invalid and 'a' is 1.

-Evan