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Date:   Wed, 3 Mar 2021 15:45:23 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Jacob Pan <jacob.jun.pan@linux.intel.com>
CC:     Liu Yi L <yi.l.liu@intel.com>, <alex.williamson@redhat.com>,
        <eric.auger@redhat.com>, <baolu.lu@linux.intel.com>,
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        <iommu@lists.linux-foundation.org>, <kvm@vger.kernel.org>,
        <Lingshan.Zhu@intel.com>, <vivek.gautam@arm.com>
Subject: Re: [Patch v8 04/10] vfio/type1: Support binding guest page tables
 to PASID
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On Wed, Mar 03, 2021 at 11:42:12AM -0800, Jacob Pan wrote:
> Hi Jason,
> 
> On Tue, 2 Mar 2021 13:15:51 -0400, Jason Gunthorpe <jgg@nvidia.com> wrote:
> 
> > On Tue, Mar 02, 2021 at 09:13:19AM -0800, Jacob Pan wrote:
> > > Hi Jason,
> > > 
> > > On Tue, 2 Mar 2021 08:56:28 -0400, Jason Gunthorpe <jgg@nvidia.com>
> > > wrote: 
> > > > On Wed, Mar 03, 2021 at 04:35:39AM +0800, Liu Yi L wrote:  
> > > > >  
> > > > > +static int vfio_dev_bind_gpasid_fn(struct device *dev, void *data)
> > > > > +{
> > > > > +	struct domain_capsule *dc = (struct domain_capsule *)data;
> > > > > +	unsigned long arg = *(unsigned long *)dc->data;
> > > > > +
> > > > > +	return iommu_uapi_sva_bind_gpasid(dc->domain, dev,
> > > > > +					  (void __user *)arg);    
> > > > 
> > > > This arg buisness is really tortured. The type should be set at the
> > > > ioctl, not constantly passed down as unsigned long or worse void *.
> > > > 
> > > > And why is this passing a __user pointer deep into an iommu_* API??
> > > >   
> > > The idea was that IOMMU UAPI (not API) is independent of VFIO or other
> > > user driver frameworks. The design is documented here:
> > > Documentation/userspace-api/iommu.rst
> > > IOMMU UAPI handles the type and sanitation of user provided data.  
> > 
> > Why? If it is uapi it has defined types and those types should be
> > completely clear from the C code, not obfuscated.
> > 
> From the user's p.o.v., it is plain c code nothing obfuscated. As for
> kernel handling of the data types, it has to be answered by the bigger
> question of how we deal with sharing IOMMU among multiple subsystems with
> UAPIs.
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As I said, don't obfuscate types like this in the kernel. It is not
good style.

> However, IOMMU is a system device which has little value to be exposed to
> the userspace. Not to mention the device-IOMMU affinity/topology. VFIO
> nicely abstracts IOMMU from the userspace, why do we want to reverse that?

The other patch was talking about a /dev/ioasid - why can't this stuff
be run over that?

Jason

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Date: Wed, 3 Mar 2021 15:45:23 -0400
From: Jason Gunthorpe <jgg@nvidia.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Subject: Re: [Patch v8 04/10] vfio/type1: Support binding guest page tables
 to PASID
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On Wed, Mar 03, 2021 at 11:42:12AM -0800, Jacob Pan wrote:
> Hi Jason,
> 
> On Tue, 2 Mar 2021 13:15:51 -0400, Jason Gunthorpe <jgg@nvidia.com> wrote:
> 
> > On Tue, Mar 02, 2021 at 09:13:19AM -0800, Jacob Pan wrote:
> > > Hi Jason,
> > > 
> > > On Tue, 2 Mar 2021 08:56:28 -0400, Jason Gunthorpe <jgg@nvidia.com>
> > > wrote: 
> > > > On Wed, Mar 03, 2021 at 04:35:39AM +0800, Liu Yi L wrote:  
> > > > >  
> > > > > +static int vfio_dev_bind_gpasid_fn(struct device *dev, void *data)
> > > > > +{
> > > > > +	struct domain_capsule *dc = (struct domain_capsule *)data;
> > > > > +	unsigned long arg = *(unsigned long *)dc->data;
> > > > > +
> > > > > +	return iommu_uapi_sva_bind_gpasid(dc->domain, dev,
> > > > > +					  (void __user *)arg);    
> > > > 
> > > > This arg buisness is really tortured. The type should be set at the
> > > > ioctl, not constantly passed down as unsigned long or worse void *.
> > > > 
> > > > And why is this passing a __user pointer deep into an iommu_* API??
> > > >   
> > > The idea was that IOMMU UAPI (not API) is independent of VFIO or other
> > > user driver frameworks. The design is documented here:
> > > Documentation/userspace-api/iommu.rst
> > > IOMMU UAPI handles the type and sanitation of user provided data.  
> > 
> > Why? If it is uapi it has defined types and those types should be
> > completely clear from the C code, not obfuscated.
> > 
> From the user's p.o.v., it is plain c code nothing obfuscated. As for
> kernel handling of the data types, it has to be answered by the bigger
> question of how we deal with sharing IOMMU among multiple subsystems with
> UAPIs.
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	 From:To:CC:Subject:Message-ID:References:Content-Type:
	 Content-Disposition:In-Reply-To:X-ClientProxiedBy:MIME-Version:
	 X-MS-Exchange-MessageSentRepresentingType:X-Header;
	b=Jyxs64SLsrGfmoPaAHc58Bpre6/+/wDGs/dAMDviWTwRc+2Miw0+jOBGQXNLz/lE8
	 KbS/K02BZmgn/jJwl994Po6nS3kGgTg0of6AFSd2MqtaZPMx+aMJ3prec9hwpHaXmN
	 SiosC+FviWoFQHmEvWSQoVeEMS093zQ+sjcsymUkMXYHGYQRqebW101Mii6/0hT3iv
	 Su4YvAyaKmOpWT8sayI4K0ICIhRxWAOT5P78FrXofij9o3X9T9F/9Bo+S5BWSCMzMr
	 +dM2KsTd4Ecac9hNemigs87T/tiCh50XaZoc8WMRFWGYMpha6KFdCV5wWL0Yzauyt7
	 H7uCWkmg/QTCw==

As I said, don't obfuscate types like this in the kernel. It is not
good style.

> However, IOMMU is a system device which has little value to be exposed to
> the userspace. Not to mention the device-IOMMU affinity/topology. VFIO
> nicely abstracts IOMMU from the userspace, why do we want to reverse that?

The other patch was talking about a /dev/ioasid - why can't this stuff
be run over that?

Jason
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