linux-kernel - Re: [PATCH 4/5] PCI/vmd: Stop overriding dma_map

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Message-ID: <20190829141453.GC18677@lst.de>
Date:   Thu, 29 Aug 2019 16:14:53 +0200
From:   Christoph Hellwig <hch@....de>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        "Derrick, Jonathan" <jonathan.derrick@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "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 4/5] PCI/vmd: Stop overriding dma_map_ops

On Wed, Aug 28, 2019 at 09:01:06AM -0600, Keith Busch wrote:
> On Wed, Aug 28, 2019 at 07:14:42AM -0700, 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.
> 
> This tests out fine on VMD hardware, but it's quite different than the
> previous patch. In v1, the original dev was used in iommu_need_mapping(),
> but this time it's the vmd device. Is this still using the actual device's
> DMA mask then?

True.  But then again I think the old one was broken as well, as it
will pass the wrong dev to identity_mapping() or
iommu_request_dma_domain_for_dev.   So I guess I'll need to respin it
a bit to do the work in iommu_need_mapping again, and then factor
that one to make it obvious what device we deal with.