[jeff-davis]

So RISC-V is open, but the extensions are often proprietary?

And what does virtualization even mean when extensions proliferate? Will you need a distinct physical machine for each combination of extensions that someone might want to virtualize?

[pavehawk2007]

Properitary = custom...it just means that it's particular to a single chip manufacturer. They have set aside opcode space for doing just that. RISC-V can't forecast everything someone will want to do with a RISC-V chip. Instead, they promise not to use those opcodes so that it won't conflict with a chip manufacturer's "custom" instructions.

Here's a post from SiFive talking specifically about DSAs in RISC-V: https://www.sifive.com/blog/part-1-fast-access-to-accelerato...

In terms of virtualization, most of the extensions can be emulated. This happens a lot with the hodgepodge of extensions laid on Intel/AMD, such as SSE, SSSE, AVX, and so forth. Just because the underlying physical machine doesn't necessarily support it doesn't mean that the guest can't. At the operating system level, the OS can read the misa (Machine ISA) register to see which extensions are supported, and emulate those which are not. I don't think RISC-V solves the issues that virtualizing Intel/AMD also suffer--and I don't think that's really their goal.

In terms of the host, if there is an extension that cannot be emulated, then yes, I would think you'd need the physical machine to be able to support it.