Re: [PATCH v11 10/14] riscv: hwprobe: Add thead vendor extension probing

[Charlie Jenkins <charlie@rivosinc.com>]

On Thu, Nov 14, 2024 at 10:44:37AM +0800, Yangyu Chen wrote:
> 
> 
> On 11/14/24 10:21, Charlie Jenkins wrote:
> > Add a new hwprobe key "RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0" which
> > allows userspace to probe for the new RISCV_ISA_VENDOR_EXT_XTHEADVECTOR
> > vendor extension.
> > 
> 
> Hi Charlie,
> 
> How about changing the name of the key from
> "RISCV_ISA_VENDOR_EXT_XTHEADVECTOR" to "RISCV_HWPROBE_KEY_VENDOR_EXT_0" and
> use marchid to identify what the vendor is, each vendor will have its own
> bit definition in this value. So we can avoid adding so many hwprobe keys
> for each vendor in the future.
> 
> I proposed a commit here: https://github.com/cyyself/linux/commit/36390645d85d1ac75dd71172f167719df4297f59

I actually originally had this in one of my first versions of this
series but was convinced by Conor to change it. The problem with it was
that tying vendor extensions to mvendorid means that it is enforced by
the kernel that vendors cannot share vendor extensions. It is possible
for vendor A to purchase IP that contains a vendor extension from vendor
B. This vendor extension should work on platforms created by vendor A
and vendor B. However, vendor A and vendor B have different mvendorids,
so the kernel can't support this if it is tied to mvendorid.  It could
be solved by duplicating every extension that vendors have, but then
userspace software would have to keep in mind the mvendorid they are
running on and check the different extensions for the different vendors
even though the implementation of the extension is the same.

The original conversation where Conor and I agreed that it was better to
have vendor extensions not rely on mvendorid:

https://lore.kernel.org/linux-riscv/20240416-husband-flavored-96c1dad58b6e@wendy/

> 
> > This new key will allow userspace code to probe for which thead vendor
> > extensions are supported. This API is modeled to be consistent with
> > RISCV_HWPROBE_KEY_IMA_EXT_0. The bitmask returned will have each bit
> > corresponding to a supported thead vendor extension of the cpumask set.
> > Just like RISCV_HWPROBE_KEY_IMA_EXT_0, this allows a userspace program
> > to determine all of the supported thead vendor extensions in one call.
> > 
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > Reviewed-by: Evan Green <evan@rivosinc.com>
> > ---
> >   arch/riscv/include/asm/hwprobe.h                   |  3 +-
> >   .../include/asm/vendor_extensions/thead_hwprobe.h  | 19 +++++++++++
> >   .../include/asm/vendor_extensions/vendor_hwprobe.h | 37 ++++++++++++++++++++++
> >   arch/riscv/include/uapi/asm/hwprobe.h              |  3 +-
> >   arch/riscv/include/uapi/asm/vendor/thead.h         |  3 ++
> >   arch/riscv/kernel/sys_hwprobe.c                    |  5 +++
> >   arch/riscv/kernel/vendor_extensions/Makefile       |  1 +
> >   .../riscv/kernel/vendor_extensions/thead_hwprobe.c | 19 +++++++++++
> >   8 files changed, 88 insertions(+), 2 deletions(-)
> > 
>