Re: [RFC PATCH kernel] vfio-pci: Allow mapping MSIX BAR

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 29, 2017 at 06:58:11PM +1100, Alexey Kardashevskiy wrote:
> On 29/11/17 16:17, David Gibson wrote:
> > On Wed, Nov 29, 2017 at 03:38:30PM +1100, Alexey Kardashevskiy wrote:
> >> On 29/11/17 14:57, David Gibson wrote:
> >>> On Wed, Nov 29, 2017 at 02:25:43PM +1100, Alexey Kardashevskiy wrote:
> >>>> On 22/11/17 17:51, David Gibson wrote:
> >>>>> On Tue, Nov 21, 2017 at 10:14:45PM -0700, Alex Williamson wrote:
> >>>>>> On Wed, 22 Nov 2017 15:44:55 +1100
> >>>>>> David Gibson <david@gibson.dropbear.id.au> wrote:
> >>>>>>
> >>>>>>> On Tue, Nov 21, 2017 at 09:28:46PM -0700, Alex Williamson wrote:
> >>>>>>>> On Wed, 22 Nov 2017 15:09:32 +1100
> >>>>>>>> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> >>>>>>>>   
> >>>>>>>>> By default VFIO disables mapping of MSIX BAR to the userspace as
> >>>>>>>>> the userspace may program it in a way allowing spurious interrupts;
> >>>>>>>>> instead the userspace uses the VFIO_DEVICE_SET_IRQS ioctl.
> >>>>>>>>>
> >>>>>>>>> This works fine as long as the system page size equals to the MSIX
> >>>>>>>>> alignment requirement which is 4KB. However with a bigger page size
> >>>>>>>>> the existing code prohibits mapping non-MSIX parts of a page with MSIX
> >>>>>>>>> structures so these parts have to be emulated via slow reads/writes on
> >>>>>>>>> a VFIO device fd. If these emulated bits are accessed often, this has
> >>>>>>>>> serious impact on performance.
> >>>>>>>>>
> >>>>>>>>> This adds an ioctl to the vfio-pci device which hides the sparse
> >>>>>>>>> capability and allows the userspace to map a BAR with MSIX structures.  
> >>>>>>>>
> >>>>>>>> So the user is in control of telling the kernel whether they're allowed
> >>>>>>>> to mmap the msi-x vector table.  That makes absolutely no sense.  If
> >>>>>>>> you're trying to figure out how userspace knows whether to implicitly
> >>>>>>>> avoid mmap'ing the msix region, I think there are far better ways in
> >>>>>>>> the existing region info ioctl.  We could use a flag, or maybe the
> >>>>>>>> existence of a capability chain pointer, or a new capability.  But
> >>>>>>>> absolutely not this.  The kernel needs to decide whether it's going to
> >>>>>>>> let the user do this, not the user.  Thanks,  
> >>>>>>>
> >>>>>>> No, it doesn't.  This is actually the approach we discussed in Prague.
> >>>>>>>
> >>>>>>> Remember that intercepting access to the MSI-X table is not a host
> >>>>>>> safety / security issue.  It's just that without that we won't wire up
> >>>>>>> the device's MSI-X vectors properly so they won't work.
> >>>>>>>
> >>>>>>> Basically the decision here is between
> >>>>>>>
> >>>>>>>    A) Allow MSI-X configuration via standard PCI mechanisms, at the
> >>>>>>>       cost of making access slow for any registers sharing a page with
> >>>>>>>       the MSI-X table.
> >>>>>>>
> >>>>>>> or
> >>>>>>>
> >>>>>>>    B) Make access to BAR registers sharing a page with the MSI-X table
> >>>>>>>       fast, at the cost of requiring some alternative mechanism to
> >>>>>>>       configure MSI-X vectors.
> >>>>>>>
> >>>>>>> And that is a tradeoff that it is reasonable for userspace to make.
> >>>>>>>
> >>>>>>> In the case of KVM guests, the decision depends entirely on the
> >>>>>>> *guest* platform.  Usually we need (A) because the guest expects to be
> >>>>>>> able to poke the MSI-X table in the usual way.  However for PAPR
> >>>>>>> guests, there's an alternative mechanism via an RTAS call, which means
> >>>>>>> we can use (B).
> >>>>>>>
> >>>>>>> The host kernel can't make this decision, because it doesn't know the
> >>>>>>> guest platform (well, KVM might, but VFIO doesn't).
> >>>>>>>
> >>>>>>> A userspace VFIO program could also elect for (B) if it does care
> >>>>>>> about performance of access to registers in the same BAR as the MSI-X
> >>>>>>> table, but doesn't need MSI-X for example.
> >>>>>>
> >>>>>> You're asking for an ioctl to allow the kernel to allow the user to
> >>>>>> mmap the page, when instead we could just allow the user to mmap the
> >>>>>> page and whether the user does that and how they make use of it is up
> >>>>>> to them...
> >>>>>
> >>>>> Duh.  Sorry.  For some reason I was thinking the magic MSI-X
> >>>>> interception was happening in the host kernel rather than in qemu.
> >>>>>
> >>>>>> I understand that there are different virtualization techniques at play
> >>>>>> here, it just doesn't seem relevant.  In the case of (A), the user can
> >>>>>> choose not to mmap the page overlapping the vector table even if the
> >>>>>> kernel allows it.  The user can also choose to mmap that page, but not
> >>>>>> use the portion overlapping the vector table.  QEMU already does this
> >>>>>> by overlaying a MemoryRegion for vector table emulation.  We might even
> >>>>>> be able to get away with mmaping that page and emulating the vector
> >>>>>> table elsewhere, which seems like the only option for a 64k page ARM
> >>>>>> system.  For (B), clearly it's just a nuisance that we can't currently
> >>>>>> mmap this page, but I still don't see how the user allowing the kernel
> >>>>>> to allow the user to mmap that page makes any sense.  I can't even
> >>>>>> describe it without it sounding ridiculous.  Thanks,
> >>>>>
> >>>>> Right.  Rethinking..  it seems to me we should just completely remove
> >>>>> the logic from the kernel banning mmap()s overlapping the MSI-X
> >>>>> table.  All it does is poorly attempt to stop the user shooting
> >>>>> themselves in the foot.
> >>>>>
> >>>>> Then we just need logic in qemu to avoid doing the overlapping memory
> >>>>> region nonsense on a per-machine basis
> >>>>
> >>>>
> >>>> So is there still any plan or we just ditch the feature? I am confused now.
> >>>
> >>> The plan is what I said above.  Remove the bogus check logic from the
> >>> kernel, then solve within qemu, by not creating the MSI-X intercept
> >>> region for pseries guests.
> >>
> >>
> >> There were 2 proposals how to do that. Both included platform code to
> >> decide whether to allow mapping or not  and  some transport to pass that
> >> enablement flag from the plafform code to the VFIO-PCI driver, one was via
> >> an IOMMU group flag, the other via a PCI bus flag. Neither was accepted so
> >> reposting those won't make any progress, what do I miss here? Was there any
> >> agreement on how to do this?
> > 
> > As we've discussed the filtering of mmap()able regions in the kernel
> > is not a security concern.  All it accomplishes is poorly trying to
> > stop userspace from shooting itself in the foot by directly mapping
> > and accessing the MSI-X table instead of using the VFIO interfaces to
> > set up MSI vectors.
> > 
> > We should simply remove - unconditionally - the checks in the kernel
> > which prevent parts of the BARs from being mapped.
> 
> 
> Just like this? And Alex is ok with that?

Yes, just like this.  I was kind of hoping Alex would express an
opinion when I mentioned it above.  But at any rate I will advocate
for this approach with him.

At least until someone points out another error in my thinking and I
try again.

> 
> 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 9ed1ecf..106b825 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -1146,22 +1146,6 @@ static int vfio_pci_mmap(void *device_data, struct
> vm_area_struct *vma)
>         if (req_start + req_len > phys_len)
>                 return -EINVAL;
> 
> -       if (!vdev->msix_mmap && index == vdev->msix_bar) {
> -               /*
> -                * Disallow mmaps overlapping the MSI-X table; users don't
> -                * get to touch this directly.  We could find somewhere
> -                * else to map the overlap, but page granularity is only
> -                * a recommendation, not a requirement, so the user needs
> -                * to know which bits are real.  Requiring them to mmap
> -                * around the table makes that clear.
> -                */
> -
> -               /* If neither entirely above nor below, then it overlaps */
> -               if (!(req_start >= vdev->msix_offset + vdev->msix_size ||
> -                     req_start + req_len <= vdev->msix_offset))
> -                       return -EINVAL;
> -       }
> 
> 
> 
> 




-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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