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 [off-list ref] wrote:

quoted

On 18/10/2023 03:45, Jerry Shih wrote:

quoted

On Oct 17, 2023, at 21:14, Clément Léger [off-list ref] wrote:

quoted

@@ -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