linux-kernel - Re: NVMe vs DMA addressing limitations

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Date:   Tue, 10 Jan 2017 11:54:26 +0100
From:   Arnd Bergmann <arnd@...aro.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Simon Horman <horms@...ge.net.au>, linux-pci@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        artemi.ivanov@...entembedded.com,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        linux-nvme@...ts.infradead.org
Subject: Re: NVMe vs DMA addressing limitations

On Tuesday, January 10, 2017 8:07:20 AM CET Christoph Hellwig wrote:
> On Tue, Jan 10, 2017 at 09:47:21AM +0300, Nikita Yushchenko wrote:
> > I'm now working with HW that:
> > - is now way "low end" or "obsolete", it has 4G of RAM and 8 CPU cores,
> > and is being manufactured and developed,
> > - has 75% of it's RAM located beyond first 4G of address space,
> > - can't physically handle incoming PCIe transactions addressed to memory
> > beyond 4G.
> 
> It might not be low end or obselete, but it's absolutely braindead.
> Your I/O performance will suffer badly for the life of the platform
> because someone tries to save 2 cents, and there is not much we can do
> about it.

Unfortunately it is a common problem for arm64 chips that were designed
by taking a 32-bit SoC and replacing the CPU core. The swiotlb is the
right workaround for this, and I think we all agree that we should
just make it work correctly.

	Arnd