linux-kernel - Re: [PATCH v5 0/5] cover-letter: Simplify vfio_iommu

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Date:   Wed, 6 Jul 2022 10:53:52 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     <joro@...tes.org>, <will@...nel.org>, <marcan@...can.st>,
        <sven@...npeter.dev>, <robin.murphy@....com>,
        <robdclark@...il.com>, <baolu.lu@...ux.intel.com>,
        <orsonzhai@...il.com>, <baolin.wang7@...il.com>,
        <zhang.lyra@...il.com>, <jean-philippe@...aro.org>,
        <jgg@...dia.com>, <kevin.tian@...el.com>,
        <suravee.suthikulpanit@....com>, <alyssa@...enzweig.io>,
        <dwmw2@...radead.org>, <mjrosato@...ux.ibm.com>,
        <gerald.schaefer@...ux.ibm.com>, <thierry.reding@...il.com>,
        <vdumpa@...dia.com>, <jonathanh@...dia.com>, <cohuck@...hat.com>,
        <thunder.leizhen@...wei.com>, <christophe.jaillet@...adoo.fr>,
        <chenxiang66@...ilicon.com>, <john.garry@...wei.com>,
        <yangyingliang@...wei.com>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCH v5 0/5] cover-letter: Simplify vfio_iommu_type1
 attach/detach routine

On Wed, Jul 06, 2022 at 11:42:17AM -0600, Alex Williamson wrote:

> On Fri, 1 Jul 2022 14:44:50 -0700
> Nicolin Chen <nicolinc@...dia.com> wrote:
> 
> > This is a preparatory series for IOMMUFD v2 patches. It enforces error
> > code -EMEDIUMTYPE in iommu_attach_device() and iommu_attach_group() when
> > an IOMMU domain and a device/group are incompatible. It also drops the
> > useless domain->ops check since it won't fail in current environment.
> >
> > These allow VFIO iommu code to simplify its group attachment routine, by
> > avoiding the extra IOMMU domain allocations and attach/detach sequences
> > of the old code.
> >
> > Worths mentioning the exact match for enforce_cache_coherency is removed
> > with this series, since there's very less value in doing that as KVM will
> > not be able to take advantage of it -- this just wastes domain memory.
> > Instead, we rely on Intel IOMMU driver taking care of that internally.
> >
> > This is on github:
> > https://github.com/nicolinc/iommufd/commits/vfio_iommu_attach
> 
> How do you foresee this going in, I'm imagining Joerg would merge the
> first patch via the IOMMU tree and provide a topic branch that I'd
> merge into the vfio tree along with the remaining patches.  Sound
> right?  Thanks,

We don't have any build dependency between the IOMMU change and
VFIO changes, yet, without the IOMMU one, any iommu_attach_group()
failure now would be a hard failure without a chance falling back
to a new_domain, which is slightly different from the current flow.

For a potential existing use case that relies on reusing existing
domain, I think it'd be safer to have Joerg acking the first change
so you merge them all? Thank!