linux-kernel - Re: [PATCH] vmd: Stop overriding dma_map

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Message-ID: <20190827215621.GB23412@localhost.localdomain>
Date:   Tue, 27 Aug 2019 15:56:22 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     "Derrick, Jonathan" <jonathan.derrick@...el.com>
Cc:     "hch@....de" <hch@....de>, "x86@...nel.org" <x86@...nel.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "Busch, Keith" <keith.busch@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vmd: Stop overriding dma_map_ops

On Tue, Aug 27, 2019 at 11:24:05AM -0700, Derrick, Jonathan wrote:
> On Mon, 2019-08-26 at 17:06 +0200, Christoph Hellwig wrote:
> > With a little tweak to the intel-iommu code we should be able to work
> > around the VMD mess for the requester IDs without having to create giant
> > amounts of boilerplate DMA ops wrapping code.  The other advantage of
> > this scheme is that we can respect the real DMA masks for the actual
> > devices, and I bet it will only be a matter of time until we'll see the
> > first DMA challeneged NVMe devices.
> > 
> > The only downside is that we can't offer vmd as a module given that
> > intel-iommu calls into it.  But the driver only has about 700 lines
> > of code, so this should not be a major issue.
> If we're going to remove its ability to be a module, and given its
> small size, could we make this default =y?
> 
> Otherwise we risk breaking platforms which have it enabled with OSVs
> who miss enabling it

Can we keep this as a module if we stick the remapping struct device
in pci_sysdata instead of going through the vmd driver to get it?

Otherwise, very happy to see this dma wrapping go away.