[PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the dma-iommu api
tmurphy at arista.com
Mon Jun 3 04:27:37 PDT 2019
On Mon, Jun 3, 2019 at 11:52 AM Joerg Roedel <joro at 8bytes.org> wrote:
> Hi Tom,
> On Mon, May 06, 2019 at 07:52:02PM +0100, Tom Murphy wrote:
> > Convert the AMD iommu driver to the dma-iommu api. Remove the iova
> > handling and reserve region code from the AMD iommu driver.
> Thank you for your work on this! I appreciate that much, but I am not
> sure we are ready to make that move for the AMD and Intel IOMMU drivers
> My main concern right now is that these changes will add a per-page
> table lock into the fast-path for dma-mapping operations. There has been
> much work in the past to remove all locking from these code-paths and
> make it scalable on x86.
Where is the locking introduced? intel doesn't use a lock in it's
because it cleverly uses cmpxchg64 to avoid using locks:
And the locking in AMD's iommu_map function can be removed (and i have
removed it in my patch set) because it does that same thing as intel:
Is there something I'm missing?
> The dma-ops implementations in the x86 IOMMU drivers have the benefit
> that they can call their page-table manipulation functions directly and
> without locks, because they can make the necessary assumptions. The
> IOMMU-API mapping/unmapping path can't make these assumptions because it
> is also used for non-DMA-API use-cases.
> So before we can move the AMD and Intel drivers to the generic DMA-API
> implementation we need to solve this problem to not introduce new
> scalability regressions.
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