On Wed, Jul 01, 2020 at 10:34:53AM -0700, Stefano Stabellini wrote: > 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 :-( Not really, most of virtio implementations are in full control of memory, being part of the hypervisor. > > > > 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. I'll need to think about it. Sounds reasonable on the surface ... -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
