linux-kernel - Re: [PATCH] riscv: Support non-coherency memory model

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Date:   Wed, 24 Apr 2019 16:23:06 +0200
From:   Christoph Hellwig <hch@....de>
To:     Gary Guo <gary@...yguo.net>
Cc:     Guo Ren <guoren@...nel.org>, Christoph Hellwig <hch@....de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Andrew Waterman <andrew@...ive.com>,
        Arnd Bergmann <arnd@...db.de>, Anup Patel <anup.patel@....com>,
        Xiang Xiaoyan <xiaoyan_xiang@...ky.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Vincent Chen <vincentc@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        "ren_guo@...ky.com" <ren_guo@...ky.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Scott Wood <swood@...hat.com>,
        "tech-privileged@...ts.riscv.org" <tech-privileged@...ts.riscv.org>
Subject: Re: [PATCH] riscv: Support non-coherency memory model

On Wed, Apr 24, 2019 at 12:45:56PM +0000, Gary Guo wrote:
> The RISC-V privileged spec is explicitly designed to allow the 
> techniques described above (this is the sole purpose of MSTATUS.TVM). It 
> might be as high performance as a hardware with H-extension, but is 
> definitely a legit use case. In fact, it is vital for use cases like 
> recursive virtualization.
> 
> Also, I believe the PTE format of RISC-V is already frozen -- therefore 
> it is impossible now to merge GLOBAL and USER bit, nor to replace RSW 
> bit with another bit.

Yes, I do not think we can just repurpose a bit.  Even using a currently
unused one would require some gymnastics.

That being said IFF we want to support non-coherent DMA (and I think we
do as people glue together their SOCs using shoestring and paper clips,
as already demonstrated by Andes and C-SKY in RISC-V space, and most
arm, mips and ppc SOCs) we need something like this flag.  The current
RISC-V method that only allows M-mode to set up such attributes on
a small number or PMP regions just doesn't work well with the way how
Linux and most non-trivial OSes implement DMA memory allocations.

Note that I said well - in theory we can have a firmware provided
uncached pool - that is what Linux does on most nommu (that is without
pagetables) ports, but the fixed sized pool really does suck and will
make users very unhappy.