linux-kernel - Re: [PATCH v5 03/15] iommu/smmu: Report empty domain nesting info

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Date:   Thu, 16 Jul 2020 17:39:59 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     Will Deacon <will@...nel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "Wu, Hao" <hao.wu@...el.com>,
        "stefanha@...il.com" <stefanha@...il.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v5 03/15] iommu/smmu: Report empty domain nesting info

On Tue, Jul 14, 2020 at 10:12:49AM +0000, Liu, Yi L wrote:
> > Have you verified that this doesn't break the existing usage of
> > DOMAIN_ATTR_NESTING in drivers/vfio/vfio_iommu_type1.c?
> 
> I didn't have ARM machine on my hand. But I contacted with Jean
> Philippe, he confirmed no compiling issue. I didn't see any code
> getting DOMAIN_ATTR_NESTING attr in current drivers/vfio/vfio_iommu_type1.c.
> What I'm adding is to call iommu_domai_get_attr(, DOMAIN_ATTR_NESTIN)
> and won't fail if the iommu_domai_get_attr() returns 0. This patch
> returns an empty nesting info for DOMAIN_ATTR_NESTIN and return
> value is 0 if no error. So I guess it won't fail nesting for ARM.

I confirm that this series doesn't break the current support for
VFIO_IOMMU_TYPE1_NESTING with an SMMUv3. That said...

If the SMMU does not support stage-2 then there is a change in behavior
(untested): after the domain is silently switched to stage-1 by the SMMU
driver, VFIO will now query nesting info and obtain -ENODEV. Instead of
succeding as before, the VFIO ioctl will now fail. I believe that's a fix
rather than a regression, it should have been like this since the
beginning. No known userspace has been using VFIO_IOMMU_TYPE1_NESTING so
far, so I don't think it should be a concern.

And if userspace queries the nesting properties using the new ABI
introduced in this patchset, it will obtain an empty struct. I think
that's acceptable, but it may be better to avoid adding the nesting cap if
@format is 0?

Thanks,
Jean

> 
> @Eric, how about your opinion? your dual-stage vSMMU support may
> also share the vfio_iommu_type1.c code.
> 
> Regards,
> Yi Liu
> 
> > Will