linux-kernel - Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID

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Date:   Tue, 12 Oct 2021 09:33:46 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     David Gibson <david@...son.dropbear.id.au>,
        Liu Yi L <yi.l.liu@...el.com>, alex.williamson@...hat.com,
        hch@....de, jasowang@...hat.com, joro@...tes.org,
        kevin.tian@...el.com, parav@...lanox.com, lkml@...ux.net,
        pbonzini@...hat.com, lushenming@...wei.com, eric.auger@...hat.com,
        corbet@....net, ashok.raj@...el.com, yi.l.liu@...ux.intel.com,
        jun.j.tian@...el.com, hao.wu@...el.com, dave.jiang@...el.com,
        jacob.jun.pan@...ux.intel.com, kwankhede@...dia.com,
        robin.murphy@....com, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
        linux-kernel@...r.kernel.org, baolu.lu@...ux.intel.com,
        nicolinc@...dia.com
Subject: Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

On Mon, Oct 11, 2021 at 08:38:17PM -0300, Jason Gunthorpe wrote:
> On Mon, Oct 11, 2021 at 09:49:57AM +0100, Jean-Philippe Brucker wrote:
> 
> > Seems like we don't need the negotiation part?  The host kernel
> > communicates available IOVA ranges to userspace including holes (patch
> > 17), and userspace can check that the ranges it needs are within the IOVA
> > space boundaries. That part is necessary for DPDK as well since it needs
> > to know about holes in the IOVA space where DMA wouldn't work as expected
> > (MSI doorbells for example). 
> 
> I haven't looked super closely at DPDK, but the other simple VFIO app
> I am aware of struggled to properly implement this semantic (Indeed it
> wasn't even clear to the author this was even needed).
> 
> It requires interval tree logic inside the application which is not a
> trivial algorithm to implement in C.
> 
> I do wonder if the "simple" interface should have an option more like
> the DMA API where userspace just asks to DMA map some user memory and
> gets back the dma_addr_t to use. Kernel manages the allocation
> space/etc.

Agreed, it's tempting to use IOVA = VA but the two spaces aren't
necessarily compatible. An extension that plugs into the IOVA allocator
could be useful to userspace drivers.

Thanks,
Jean