linux-kernel - Re: [PATCH 1/1] drm/amdkfd: fix build error with AMD_IOMMU

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Date:   Tue, 9 Mar 2021 12:59:47 -0500
From:   Alex Deucher <alexdeucher@...il.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Felix Kuehling <felix.kuehling@....com>,
        Arnd Bergmann <arnd@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Will Deacon <will@...nel.org>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH 1/1] drm/amdkfd: fix build error with AMD_IOMMU_V2=m

On Tue, Mar 9, 2021 at 12:55 PM Jean-Philippe Brucker
<jean-philippe@...aro.org> wrote:
>
> Hi Felix,
>
> On Tue, Mar 09, 2021 at 11:30:19AM -0500, Felix Kuehling wrote:
> > > I think the proper fix would be to not rely on custom hooks into a particular
> > > IOMMU driver, but to instead ensure that the amdgpu driver can do everything
> > > it needs through the regular linux/iommu.h interfaces. I realize this
> > > is more work,
> > > but I wonder if you've tried that, and why it didn't work out.
> >
> > As far as I know this hasn't been tried. I see that intel-iommu has its
> > own SVM thing, which seems to be similar to what our IOMMUv2 does. I
> > guess we'd have to abstract that into a common API.
>
> The common API was added in 26b25a2b98e4 and implemented by the Intel
> driver in 064a57d7ddfc. To support it an IOMMU driver implements new IOMMU
> ops:
>         .dev_has_feat()
>         .dev_feat_enabled()
>         .dev_enable_feat()
>         .dev_disable_feat()
>         .sva_bind()
>         .sva_unbind()
>         .sva_get_pasid()
>
> And a device driver calls iommu_dev_enable_feature(IOMMU_DEV_FEAT_SVA)
> followed by iommu_sva_bind_device().
>
> If I remember correctly the biggest obstacle for KFD is the PASID
> allocation, done by the GPU driver instead of the IOMMU driver, but there
> may be others.

IIRC, we tried to make the original IOMMUv2 functionality generic but
other vendors were not interested at the time, so it ended up being
AMD specific and since nothing else was using the pasid allocations we
put them in the GPU driver.  I guess if this is generic now, it could
be moved to a common API and taken out of the driver.

Alex