[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] PCI Passthrough ARM Design : Draft1
On Wed, 2015-06-17 at 14:40 +0100, Stefano Stabellini wrote: > On Wed, 17 Jun 2015, Ian Campbell wrote: > > On Wed, 2015-06-17 at 13:53 +0100, Stefano Stabellini wrote: > > > On Wed, 17 Jun 2015, Ian Campbell wrote: > > > > On Tue, 2015-06-16 at 18:16 +0100, Stefano Stabellini wrote: > > > > > I wrote this before reading the rest of the thread with your > > > > > alternative > > > > > suggestion. Both approaches can work, maybe it is true that having > > > > > the > > > > > guest requesting mappings is better. And we should certainly do the > > > > > same > > > > > thing for PVH and ARM guests. > > > > > > > > > > If we have the guest requesting the mapping, obviously the hypercall > > > > > implementation should check whether mapping the memory region has been > > > > > previously allowed by the toolstack, that should still call > > > > > xc_domain_iomem_permission. > > > > > > > > Whoever makes the initial decision/setup we still need to decide what to > > > > do about reads and writes to the device's BAR registers. Who deals with > > > > these and how, and if writes are allowed and if so how is this then > > > > reflected in the guest p2m. > > > > > > > > I would very much prefer ARM and x86 PVH to use the same approach to > > > > this. > > > > > > > > For x86 HVM I believe QEMU takes care of this, by emulating the > > > > reads/writes to CFG space and making hypercalls (domctls?) to update the > > > > p2m. There is no QEMU to do this in the ARM or x86 PVH cases. > > > > > > > > For x86 PV I believe pciback deals with the register reads/writes but it > > > > doesn't need to do anything wrt the p2m because that is a guest internal > > > > construct. This obviously doesn't apply to ARM or x86 PVH either since > > > > the p2m is managed by Xen in both those cases. > > > > > > > > So it seems that neither of the existing solutions are a good match for > > > > either ARM or x86 PVH. > > > > > > > > _If_ it were permissible to implement all BAR registers as r/o then this > > > > might simplify things, however I suspect this is not something we can > > > > get away with in the general case (i.e. a driver for a given PCI device > > > > _knows_ that BAR<N> is writeable...). > > > > > > > > So I think we do need to allow guest writes to the BAR registers. I > > > > think it would be a bit strange (or even open potentially subtle > > > > (security?) issues) to require the guest to write the BAR register and > > > > to update the p2m to match as well. > > > > > > Why would it be a security risk? > > > > No idea, it seemed like a vague possibility. I did say "(security?)" not > > "SECURITY" ;-) > > > > > I don't think it is important that the > > > interface between pcifront and pciback refrects the hardware interface. > > > But it is important that the interface is documented and complies to the > > > expected behaviour. > > > > > > I think that if we go with XENMEM_add_to_physmap_range, it would be OK > > > for pcifront to both write to the vBAR (actually writing to the ring to > > > pciback), then call XENMEM_add_to_physmap_range. > > > > > > On the other hand, if we go with XEN_DOMCTL_memory_mapping, I would make > > > the virtual BAR read-only, that is less preferable but still acceptable, > > > as long as we document it. > > > > Given that this appears to be how pv pci works today I think this turns > > out to be preferable and saves us having to argue the toss about who > > should do the p2m updates on a BAR update. > > > > Which would point towards the toolstack doing the layout and telling > > pciback somehow what the bars for each device are and that's it. > > > > (making most of the rest of your reply moot, if you think its really > > important to let guests move BARs about I can go through it again) > > I think is unimportant, in fact having the guest setup the mappings was > my original suggestion. I can't reconcile the two halves of this statement, are you saying you are still in favour of the guest dealing with the mappings or not? _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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