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

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Date:   Thu, 16 Jul 2020 22:38:17 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     Will Deacon <will@...nel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...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

Hi Jean,

On 7/16/20 5:39 PM, Jean-Philippe Brucker wrote:
> 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.
But as Yi mentioned ealier, in the current vfio code there is no
DOMAIN_ATTR_NESTING query yet. In my SMMUV3 nested stage series, I added
such a query in vfio-pci.c to detect if I need to expose a fault region
but I already test both the returned value and the output arg. So to me
there is no issue with that change.
> 
> 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?
agreed

Thanks

Eric
> 
> 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
>