linux-kernel - Re: [RFC PATCH v2 06/11] riscv: pgtable: Add DMA

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Date:   Mon, 7 Jun 2021 08:19:18 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nick Kossifidis <mick@....forth.gr>
Cc:     guoren@...nel.org, anup.patel@....com, palmerdabbelt@...gle.com,
        arnd@...db.de, wens@...e.org, maxime@...no.tech,
        drew@...gleboard.org, liush@...winnertech.com,
        lazyparser@...il.com, wefu@...hat.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-sunxi@...ts.linux.dev,
        Guo Ren <guoren@...ux.alibaba.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [RFC PATCH v2 06/11] riscv: pgtable: Add DMA_COHERENT with
 custom PTE attributes

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.