On Wed, 1 Jul 2020, Christoph Hellwig wrote: > On Mon, Jun 29, 2020 at 04:46:09PM -0700, Stefano Stabellini wrote: > > > I could imagine some future Xen hosts setting a flag somewhere in the > > > platform capability saying "no xen specific flag, rely on > > > "VIRTIO_F_ACCESS_PLATFORM". Then you set that accordingly in QEMU. > > > How about that? > > > > Yes, that would be fine and there is no problem implementing something > > like that when we get virtio support in Xen. Today there are still no > > virtio interfaces provided by Xen to ARM guests (no virtio-block/net, > > etc.) > > > > In fact, in both cases we are discussing virtio is *not* provided by > > Xen; it is a firmware interface to something entirely different: > > > > 1) virtio is used to talk to a remote AMP processor (RPMesg) > > 2) virtio is used to talk to a secure-world firmware/OS (Trusty) > > > > VIRTIO_F_ACCESS_PLATFORM is not set by Xen in these cases but by RPMesg > > and by Trusty respectively. I don't know if Trusty should or should not > > set VIRTIO_F_ACCESS_PLATFORM, but I think Linux should still work > > without issues. > > > > Any virtio implementation that is not in control of the memory map > (aka not the hypervisor) absolutely must set VIRTIO_F_ACCESS_PLATFORM, > else it is completely broken.
Lots of broken virtio implementations out there it would seem :-( > > The xen_domain() check in Linux makes it so that vring_use_dma_api > > returns the opposite value on native Linux compared to Linux as Xen/ARM > > DomU by "accident". By "accident" because there is no architectural > > reason why Linux Xen/ARM DomU should behave differently compared to > > native Linux in this regard. > > > > I hope that now it is clearer why I think the if (xen_domain()) check > > needs to be improved anyway, even if we fix generic dma_ops with virtio > > interfaces missing VIRTIO_F_ACCESS_PLATFORM. > > IMHO that Xen quirk should never have been added in this form.. Would you be in favor of a more flexible check along the lines of the one proposed in the patch that started this thread: if (xen_vring_use_dma()) return true; xen_vring_use_dma would be implemented so that it returns true when xen_swiotlb is required and false otherwise. _______________________________________________ iommu mailing list firstname.lastname@example.org https://lists.linuxfoundation.org/mailman/listinfo/iommu