linux-kernel - Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

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Message-ID: <cd3ff8cf-a5a7-1a92-d92a-689edcde51d6@deltatee.com>
Date:   Fri, 2 Mar 2018 15:24:36 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Keith Busch <keith.busch@...el.com>,
        Oliver OHalloran <oliveroh@....ibm.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory



On 02/03/18 02:44 PM, Benjamin Herrenschmidt wrote:
> Allright, so, I think I have a plan to fix this, but it will take a
> little bit of time.
> 
> Basically the idea is to have firmware pass to Linux a region that's
> known to not have anything in it that it can use for the vmalloc space
> rather than have linux arbitrarily cut the address space in half.
> 
> I'm pretty sure I can always find large enough "holes" in the physical
> address space that are outside of both RAM/OpenCAPI/Nvlink and
> PCIe/MMIO space. If anything, unused chip IDs. But I don't want Linux
> to have to know about the intimate HW details so I'll pass it from FW.
> 
> It will take some time to adjust Linux and get updated FW around
> though.
> 
> Once that's done, I'll be able to have the linear mapping go through
> the entire 52-bit space (minus that hole). Of course the hole need to
> be large enough to hold a vmemmap for a 52-bit space, so that's about
> 4TB. So I probably need a hole that's at least 8TB.
> 
> As for the mapping attributes, it should be easy for my linear mapping
> code to ensure anything that isn't actual RAM is mapped NC.

Very cool. I'm glad to hear you found a way to fix this.

Thanks,

Logan