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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: MWHPR11MB1886.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 3f419dad-3974-4e44-8fc8-08d9361ccf51 X-MS-Exchange-CrossTenant-originalarrivaltime: 23 Jun 2021 07:59:21.2440 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: eWByGb40t9+OHZ1oYpuYYKFKc5H2hbmNpO6ReE3RJT9BNh/hYd1OpIBGbYcDAumr8o9nlGeDB60DgfX5vLwRvA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR11MB1933 X-OriginatorOrg: intel.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: David Gibson > Sent: Thursday, June 17, 2021 11:48 AM >=20 > On Tue, Jun 08, 2021 at 10:17:56AM -0300, Jason Gunthorpe wrote: > > On Tue, Jun 08, 2021 at 12:37:04PM +1000, David Gibson wrote: > > > > > > The PPC/SPAPR support allows KVM to associate a vfio group to an > IOMMU > > > > page table so that it can handle iotlb programming from pre-registe= red > > > > memory without trapping out to userspace. > > > > > > To clarify that's a guest side logical vIOMMU page table which is > > > partially managed by KVM. This is an optimization - things can work > > > without it, but it means guest iomap/unmap becomes a hot path because > > > each map/unmap hypercall has to go > > > guest -> KVM -> qemu -> VFIO > > > > > > So there are multiple context transitions. > > > > Isn't this overhead true of many of the vIOMMUs? >=20 > Yes, but historically it bit much harder on POWER for a couple of reasons= : >=20 > 1) POWER guests *always* have a vIOMMU - the platform has no concept > of passthrough mode. We therefore had a vIOMMU implementation some > time before the AMD or Intel IOMMUs were implemented as vIOMMUs in > qemu. >=20 > 2) At the time we were implementing this the supported IOVA window for > the paravirtualized IOMMU was pretty small (1G, I think) making > vIOMMU maps and unmaps a pretty common operation. >=20 > > Can the fast path be > > generalized? >=20 > Not really. This is a paravirtualized guest IOMMU, so it's a platform > specific group of hypercalls that's being interpreted by KVM and > passed through to the IOMMU side using essentially the same backend > that that the userspace implementation would eventually get to after a > bunch more context switches. >=20 Can virtio-iommu work on PPC? iirc Jean has a plan to implement a vhost-iommu which is supposed to implement the similar in-kernel acceleration... Thanks Kevin