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Date:   Fri, 3 Apr 2020 09:34:36 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     "Liu, Yi L" <yi.l.liu@intel.com>,
        "eric.auger@redhat.com" <eric.auger@redhat.com>,
        "jacob.jun.pan@linux.intel.com" <jacob.jun.pan@linux.intel.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Tian, Jun J" <jun.j.tian@intel.com>,
        "Sun, Yi Y" <yi.y.sun@intel.com>,
        "jean-philippe@linaro.org" <jean-philippe@linaro.org>,
        "peterx@redhat.com" <peterx@redhat.com>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Wu, Hao" <hao.wu@intel.com>
Subject: Re: [PATCH v1 7/8] vfio/type1: Add VFIO_IOMMU_CACHE_INVALIDATE
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On Fri, 3 Apr 2020 06:39:22 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Friday, April 3, 2020 4:24 AM
> > 
> > On Sun, 22 Mar 2020 05:32:04 -0700
> > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> >   
> > > From: Liu Yi L <yi.l.liu@linux.intel.com>
> > >
> > > For VFIO IOMMUs with the type VFIO_TYPE1_NESTING_IOMMU, guest  
> > "owns" the  
> > > first-level/stage-1 translation structures, the host IOMMU driver has no
> > > knowledge of first-level/stage-1 structure cache updates unless the guest
> > > invalidation requests are trapped and propagated to the host.
> > >
> > > This patch adds a new IOCTL VFIO_IOMMU_CACHE_INVALIDATE to  
> > propagate guest  
> > > first-level/stage-1 IOMMU cache invalidations to host to ensure IOMMU  
> > cache  
> > > correctness.
> > >
> > > With this patch, vSVA (Virtual Shared Virtual Addressing) can be used safely
> > > as the host IOMMU iotlb correctness are ensured.
> > >
> > > Cc: Kevin Tian <kevin.tian@intel.com>
> > > CC: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > Cc: Alex Williamson <alex.williamson@redhat.com>
> > > Cc: Eric Auger <eric.auger@redhat.com>
> > > Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > > Signed-off-by: Liu Yi L <yi.l.liu@linux.intel.com>
> > > Signed-off-by: Eric Auger <eric.auger@redhat.com>
> > > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > ---
> > >  drivers/vfio/vfio_iommu_type1.c | 49  
> > +++++++++++++++++++++++++++++++++++++++++  
> > >  include/uapi/linux/vfio.h       | 22 ++++++++++++++++++
> > >  2 files changed, 71 insertions(+)
> > >
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c  
> > b/drivers/vfio/vfio_iommu_type1.c  
> > > index a877747..937ec3f 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > @@ -2423,6 +2423,15 @@ static long  
> > vfio_iommu_type1_unbind_gpasid(struct vfio_iommu *iommu,  
> > >  	return ret;
> > >  }
> > >
> > > +static int vfio_cache_inv_fn(struct device *dev, void *data)
> > > +{
> > > +	struct domain_capsule *dc = (struct domain_capsule *)data;
> > > +	struct iommu_cache_invalidate_info *cache_inv_info =
> > > +		(struct iommu_cache_invalidate_info *) dc->data;
> > > +
> > > +	return iommu_cache_invalidate(dc->domain, dev, cache_inv_info);
> > > +}
> > > +
> > >  static long vfio_iommu_type1_ioctl(void *iommu_data,
> > >  				   unsigned int cmd, unsigned long arg)
> > >  {
> > > @@ -2629,6 +2638,46 @@ static long vfio_iommu_type1_ioctl(void  
> > *iommu_data,  
> > >  		}
> > >  		kfree(gbind_data);
> > >  		return ret;
> > > +	} else if (cmd == VFIO_IOMMU_CACHE_INVALIDATE) {
> > > +		struct vfio_iommu_type1_cache_invalidate cache_inv;
> > > +		u32 version;
> > > +		int info_size;
> > > +		void *cache_info;
> > > +		int ret;
> > > +
> > > +		minsz = offsetofend(struct  
> > vfio_iommu_type1_cache_invalidate,  
> > > +				    flags);  
> > 
> > This breaks backward compatibility as soon as struct
> > iommu_cache_invalidate_info changes size by its defined versioning
> > scheme.  ie. a field gets added, the version is bumped, all existing
> > userspace breaks.  Our minsz is offsetofend to the version field,
> > interpret the version to size, then reevaluate argsz.  
> 
> btw the version scheme is challenged by Christoph Hellwig. After
> some discussions, we need your guidance how to move forward.
> Jacob summarized available options below:
> 	https://lkml.org/lkml/2020/4/2/876

Ok

> >   
> > > +
> > > +		if (copy_from_user(&cache_inv, (void __user *)arg, minsz))
> > > +			return -EFAULT;
> > > +
> > > +		if (cache_inv.argsz < minsz || cache_inv.flags)
> > > +			return -EINVAL;
> > > +
> > > +		/* Get the version of struct iommu_cache_invalidate_info */
> > > +		if (copy_from_user(&version,
> > > +			(void __user *) (arg + minsz), sizeof(version)))
> > > +			return -EFAULT;
> > > +
> > > +		info_size = iommu_uapi_get_data_size(
> > > +					IOMMU_UAPI_CACHE_INVAL,  
> > version);  
> > > +
> > > +		cache_info = kzalloc(info_size, GFP_KERNEL);
> > > +		if (!cache_info)
> > > +			return -ENOMEM;
> > > +
> > > +		if (copy_from_user(cache_info,
> > > +			(void __user *) (arg + minsz), info_size)) {
> > > +			kfree(cache_info);
> > > +			return -EFAULT;
> > > +		}
> > > +
> > > +		mutex_lock(&iommu->lock);
> > > +		ret = vfio_iommu_for_each_dev(iommu, vfio_cache_inv_fn,
> > > +					    cache_info);  
> > 
> > How does a user respond when their cache invalidate fails?  Isn't this
> > also another case where our for_each_dev can fail at an arbitrary point
> > leaving us with no idea whether each device even had the opportunity to
> > perform the invalidation request.  I don't see how we have any chance
> > to maintain coherency after this faults.  
> 
> Then can we make it simple to support singleton group only? 

Are you suggesting a single group per container or a single device per
group?  Unless we have both, aren't we always going to have this issue.
OTOH, why should a cache invalidate fail?

> > > +		mutex_unlock(&iommu->lock);
> > > +		kfree(cache_info);
> > > +		return ret;
> > >  	}
> > >
> > >  	return -ENOTTY;
> > > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > > index 2235bc6..62ca791 100644
> > > --- a/include/uapi/linux/vfio.h
> > > +++ b/include/uapi/linux/vfio.h
> > > @@ -899,6 +899,28 @@ struct vfio_iommu_type1_bind {
> > >   */
> > >  #define VFIO_IOMMU_BIND		_IO(VFIO_TYPE, VFIO_BASE + 23)
> > >
> > > +/**
> > > + * VFIO_IOMMU_CACHE_INVALIDATE - _IOW(VFIO_TYPE, VFIO_BASE + 24,
> > > + *			struct vfio_iommu_type1_cache_invalidate)
> > > + *
> > > + * Propagate guest IOMMU cache invalidation to the host. The cache
> > > + * invalidation information is conveyed by @cache_info, the content
> > > + * format would be structures defined in uapi/linux/iommu.h. User
> > > + * should be aware of that the struct  iommu_cache_invalidate_info
> > > + * has a @version field, vfio needs to parse this field before getting
> > > + * data from userspace.
> > > + *
> > > + * Availability of this IOCTL is after VFIO_SET_IOMMU.  
> > 
> > Is this a necessary qualifier?  A user can try to call this ioctl at
> > any point, it only makes sense in certain configurations, but it should
> > always "do the right thing" relative to the container iommu config.
> > 
> > Also, I don't see anything in these last few patches testing the
> > operating IOMMU model, what happens when a user calls them when not
> > using the nesting IOMMU?
> > 
> > Is this ioctl and the previous BIND ioctl only valid when configured
> > for the nesting IOMMU type?  
> 
> I think so. We should add the nesting check in those new ioctls.
> 
> >   
> > > + *
> > > + * returns: 0 on success, -errno on failure.
> > > + */
> > > +struct vfio_iommu_type1_cache_invalidate {
> > > +	__u32   argsz;
> > > +	__u32   flags;
> > > +	struct	iommu_cache_invalidate_info cache_info;
> > > +};
> > > +#define VFIO_IOMMU_CACHE_INVALIDATE      _IO(VFIO_TYPE, VFIO_BASE  
> > + 24)
> > 
> > The future extension capabilities of this ioctl worry me, I wonder if
> > we should do another data[] with flag defining that data as CACHE_INFO.  
> 
> Can you elaborate? Does it mean with this way we don't rely on iommu
> driver to provide version_to_size conversion and instead we just pass
> data[] to iommu driver for further audit?

No, my concern is that this ioctl has a single function, strictly tied
to the iommu uapi.  If we replace cache_info with data[] then we can
define a flag to specify that data[] is struct
iommu_cache_invalidate_info, and if we need to, a different flag to
identify data[] as something else.  For example if we get stuck
expanding cache_info to meet new demands and develop a new uapi to
solve that, how would we expand this ioctl to support it rather than
also create a new ioctl?  There's also a trade-off in making the ioctl
usage more difficult for the user.  I'd still expect the vfio layer to
check the flag and interpret data[] as indicated by the flag rather
than just passing a blob of opaque data to the iommu layer though.
Thanks,

Alex