Re: [PATCH v1 7/8] vfio/type1: Add VFIO_IOMMU_CACHE_INVALIDATE

Alex Williamson
2020-04-03T15:34:56Z

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