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

From: Auger Eric <eric.auger@redhat.com>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	"Liu, Yi L" <yi.l.liu@intel.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"stefanha@gmail.com" <stefanha@gmail.com>,
	"Sun, Yi Y" <yi.y.sun@intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"alex.williamson@redhat.com" <alex.williamson@redhat.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Robin Murphy <robin.murphy@arm.com>, "Wu, Hao" <hao.wu@intel.com>,
	Will Deacon <will@kernel.org>,
	"Tian, Jun J" <jun.j.tian@intel.com>
Subject: Re: [PATCH v5 03/15] iommu/smmu: Report empty domain nesting info
Date: Thu, 16 Jul 2020 22:38:17 +0200	[thread overview]
Message-ID: <f3779a69-0295-d668-5f2f-746b6ff2bdce@redhat.com> (raw)
In-Reply-To: <20200716153959.GA447208@myrica>

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
> 

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