[RFC PATCH v2 06/11] riscv: pgtable: Add DMA_COHERENT with custom PTE attributes

[Christoph Hellwig]

On Sun, Jun 06, 2021 at 08:22:14PM +0300, Nick Kossifidis wrote:
> This patch violates the Privilege Spec section 4.4.1 that clearly states:
>
> "Bits63–54 are reserved for future standard use and must be zeroed by 
> software for forward compatibility"
>
> Standard use means that valid values can only be defined by the Priv. Spec, 
> not by the vendor (otherwise they'd be marked as "custom use" or "platform 
> use"), and since they "must" be zeroed by software we 'll be violating the 
> Privilege Spec if we do otherwise.

Yes, that is why I think it is a no-go.