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 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.

-- 
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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