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. > 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.. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
