Re: VFIO no-iommu

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2015-12-18 at 10:43 +0000, Yigit, Ferruh wrote:
> On Thu, Dec 17, 2015 at 09:43:59AM -0700, Alex Williamson wrote:
> <...>
> > > > > > > 
> > > > > > > Also I need to disable VFIO_CHECK_EXTENSION ioctl,
> > > > > > > because in
> > > > > > > vfio
> > > > > > > module,
> > > > > > > container->noiommu is not set before doing a
> > > > > > > vfio_group_set_container()
> > > > > > > and vfio_for_each_iommu_driver selects wrong driver.
> > > > > > 
> > > > > > Running CHECK_EXTENSION on a container without the group
> > > > > > attached is
> > > > > > only going to tell you what extensions vfio is capable of,
> > > > > > not
> > > > > > necessarily what extensions are available to you with that
> > > > > > group.
> > > > > > Is this just a general dpdk- vfio ordering bug?
> > > > > 
> > > > > Yes, that is how VFIO was implemented in DPDK. I was under
> > > > > the
> > > > > impression that checking extension before assigning devices
> > > > > was
> > > > > the
> > > > > correct way to do things, so as to not to try anything we
> > > > > know
> > > > > would
> > > > > fail anyway. Does this imply that CHECK_EXTENSION needs to be
> > > > > called
> > > > > on both container and groups (or just on groups)?
> > > > 
> > > > Hmm, in Documentation/vfio.txt we do give the following
> > > > algorithm:
> > > > 
> > > >         if (ioctl(container, VFIO_GET_API_VERSION) !=
> > > > VFIO_API_VERSION)
> > > >                 /* Unknown API version */
> > > > 
> > > >         if (!ioctl(container, VFIO_CHECK_EXTENSION,
> > > > VFIO_TYPE1_IOMMU))
> > > >                 /* Doesn't support the IOMMU driver we want. */
> > > >         ...
> > > > 
> > > > That's just going to query each iommu driver and we can't yet
> > > > say
> > > > whether
> > > > the group the user attaches to the container later will
> > > > actually
> > > > support that
> > > > extension until we try to do it, that would come at
> > > > VFIO_SET_IOMMU.
> > > >  So is
> > > > it perhaps a vfio bug that we're not advertising no-iommu until
> > > > the
> > > > group is
> > > > attached?  After all, we are capable of it with just an empty
> > > > container, just
> > > > like we are with type1, but we're going to fail SET_IOMMU for
> > > > the
> > > > wrong
> > > > combination.
> > > >  This is exactly the sort of thing that makes me glad we
> > > > reverted
> > > > it without
> > > > feedback from a working user driver.  Thanks,
> > > 
> > > Whether it should be considered a "bug" in VFIO or "by design" is
> > > up
> > > to you, of course, but at least according to the VFIO
> > > documentation,
> > > we are meant to check for type 1 extension and then attach
> > > devices,
> > > so it would be expected to get VFIO_NOIOMMU_IOMMU marked as
> > > supported
> > > even without any devices attached to the container (just like we
> > > get
> > > type 1 as supported without any devices attached). Having said
> > > that,
> > > if it was meant to attach devices first and then check the
> > > extensions, then perhaps the documentation should also point out
> > > that
> > > fact (or perhaps I missed that detail in my readings of the docs,
> > > in
> > > which case my apologies).
> > 
> > Hi Anatoly,
> > 
> > Does the below patch make it behave more like you'd expect.  This
> > applies to v4.4-rc4, I'd fold this into the base patch if we
> > reincorporate it to a future kernel.  Thanks,
> > 
> > Alex
> > 
> > commit 88d4dcb6b77624965f0b45b5cd305a2b4a105c94
> > Author: Alex Williamson <alex.williamson@redhat.com>
> > Date:   Wed Dec 16 19:02:01 2015 -0700
> > 
> >     vfio: Fix no-iommu CHECK_EXTENSION
> >     
> >     Previously the no-iommu iommu driver was only visible when the
> >     container had an attached no-iommu group.  This means that
> >     CHECK_EXTENSION on and empty container couldn't report the
> > possibility
> >     of using VFIO_NOIOMMU_IOMMU.  We report TYPE1 whether or not
> > the user
> >     can make use of it with the group, so this is
> > inconsistent.  Add the
> >     no-iommu iommu to the list of iommu drivers when enabled via
> > module
> >     option, but skip all the others if the container is attached to
> > a
> >     no-iommu groups.  Note that tainting is now done with the
> > "unsafe"
> >     module callback rather than explictly within vfio.
> >     
> >     Also fixes module option and module description name
> > inconsistency.
> >     
> >     Also make vfio_noiommu_ops const.
> >     
> >     Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> 
> Hi Alex,
> 
> I got following crash with this update:
> 
> [  +0.046973] BUG: unable to handle kernel NULL pointer dereference
> at           (null)
> [  +0.000031] IP: [<ffffffff813b92cf>] __list_add+V0x1f/0xc0
> [  +0.000024] PGD 0 
> [  +0.000010] Oops: 0000 [#1] SMP 
> ...
> [  +0.000028] Call Trace:
> [  +0.000014]  [<ffffffff81777301>] __mutex_lock_slowpath+0x91/0x110
> [  +0.000022]  [<ffffffff817773a3>] mutex_lock+0x23/0x40
> [  +0.000020]  [<ffffffffa0370c00>]
> vfio_register_iommu_driver+0x40/0xc0 [vfio]
> [  +0.000025]  [<ffffffffa0370cd1>] noiommu_param_set+0x51/0x60
> [vfio]
> [  +0.000022]  [<ffffffff810bbe3e>] parse_args+0x1be/0x4a0
> [  +0.000021]  [<ffffffff81121ea0>] load_module+0xe30/0x2730
> [  +0.000942]  [<ffffffff8111f500>] ? __symbol_put+0x60/0x60
> [  +0.000921]  [<ffffffff81223f60>] ? kernel_read+0x50/0x80
> [  +0.000917]  [<ffffffff811239f9>] SyS_finit_module+0xb9/0xf0
> [  +0.000925]  [<ffffffff817796ae>]
> entry_SYSCALL_64_fastpath+0x12/0x71
> 
> > -static struct vfio_iommu_driver vfio_noiommu_driver = {
> > -	.ops = &vfio_noiommu_ops,
> > +static int noiommu_param_set(const char *val, const struct
> > kernel_param *kp)
> > +{
> > +	int ret;
> > +
> > +	if (!val)
> > +		val = "1";
> > +
> > +	ret = strtobool(val, kp->arg);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (noiommu)
> > +		ret =
> > vfio_register_iommu_driver(&vfio_noiommu_ops);
> 
> Is it possible that kernel_param_ops->set() called before
> module_init() that initializes the mutex.
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/
> kernel/module.c?id=refs/tags/v4.4-rc5#n3517

Yes, guess I didn't think that one through very well.  I'll try again.
 Thank you for testing, sorry it was a dud.  Thanks,

Alex