Re: [PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the dma-iommu api

From: Joerg Roedel
Date: Mon Jun 03 2019 - 06:55:47 EST

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.

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.