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@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx> 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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