Is a non-power-of-2 vlenb allowed?

[palmer-dabbelt]

This came up in passing on a QEMU review https://lore.kernel.org/r/mhng-348cd335-c03c-481a-b09f-06bb3134650f@palmer-ri-x1c9a/. I'd been assuming that vlenb is a power of two, but I don't see anything in the spec that says so. I'm not sure if I'm missing something?

[nick-knight]

It follows from the following:

Each hart supporting a vector extension defines two parameters:

1. The maximum size in bits of a vector element that any operation can produce or consume, ELEN ≥ 8, which must be a power of 2.
2. The number of bits in a single vector register, VLEN ≥ ELEN, which must be a power of 2, and must be no greater than 2^16.

[...]

The XLEN-bit-wide read-only CSR vlenb holds the value VLEN/8, i.e., the vector register length in bytes.

[aswaterman]

IOW, yes, the spec says it must be a power of two.

[palmer-dabbelt]

Thanks, I guess I just wasn't paying attention -- I was only looking for vlenb.