On Fri, Feb 07, 2020 at 03:30:55PM +1100, David Gibson wrote:
> Traditionally, virtio devices don't do DMA by the usual path on the
> guest platform. In particular they usually bypass any virtual IOMMU
> the guest has, using hypervisor magic to access untranslated guest
> physical addresses.
>
> There's now the optional iommu_platform flag which can tell virtio
> devices to use the platform's normal DMA path, including any IOMMUs.
> That flag was motiviated for the case of hardware virtio
> implementations, but there are other reasons to want it.
>
> Specifically, the fact that the virtio device doesn't use vIOMMU
> translation means that virtio devices are unsafe to pass to nested
> guests, or to use with VFIO userspace drivers inside the guest. This
> is particularly noticeable on the pseries platform which *always* has
> a guest-visible vIOMMU.
>
> Not using the normal DMA path also causes difficulties for the guest
> side driver when using the upcoming POWER Secure VMs (a.k.a. PEF).
> While it's theoretically possible to handle this on the guest side,
> it's really fiddly. Given the other problems with the non-translated
> virtio device, let's just enable vIOMMU translation for virtio devices
> by default in the pseries-5.0 (and later) machine types.
>
> Signed-off-by: David Gibson <da...@gibson.dropbear.id.au>
Worth noting that since iommu_platform is mandatory for guests,
this disables support for guests older than Linux 4.8.
> ---
> hw/ppc/spapr.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 216d3b34dc..78e031e80a 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -4518,6 +4518,7 @@ static void
> spapr_machine_5_0_class_options(MachineClass *mc)
> * default behaviour for virtio */
> static GlobalProperty compat[] = {
> { TYPE_VIRTIO_PCI, "disable-legacy", "on", },
> + { TYPE_VIRTIO_DEVICE, "iommu_platform", "on", },
> };
>
> compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
> @@ -4533,6 +4534,7 @@ static void
> spapr_machine_4_2_class_options(MachineClass *mc)
> SpaprMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
> static GlobalProperty compat[] = {
> { TYPE_VIRTIO_PCI, "disable-legacy", "auto" },
> + { TYPE_VIRTIO_DEVICE, "iommu_platform", "off", },
> };
>
> spapr_machine_5_0_class_options(mc);
> --
> 2.24.1
