Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 19 Jun 2023 11:02:12 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Baolu Lu <baolu.lu@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>
Subject: Re: Question about reserved_regions w/ Intel IOMMU
Message-ID: <ZJBf5DP60prFH5R2@nvidia.com>
References: <CAKgT0UfMeVOz6AOqSvVvzpsedGDiXCNQrjM+4KDv7qJJ1orpsw@mail.gmail.com>
 <a1cff65b-b390-3872-25b5-dd6bbfb3524c@linux.intel.com>
 <b24a6c7b-27fc-41c0-5c82-15696b4a7dc1@arm.com>
 <ZIiRK2Dzl2/9Jqle@ziepe.ca>
 <BN9PR11MB52765C24405D2475CF3CBEBE8C58A@BN9PR11MB5276.namprd11.prod.outlook.com>
 <ZIxTmGU4a5dniEY3@nvidia.com>
 <CAKgT0UfmdOOPSD5YvpHnh1A02URn9zxVLbyXJM_67On7xojLcA@mail.gmail.com>
 <520e2be4-726f-c680-c010-a308cdddbae0@arm.com>
 <ZIyxFpzh3WG+ifws@nvidia.com>
 <90823b33-1f44-8789-9a38-282407fd9f15@arm.com>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <90823b33-1f44-8789-9a38-282407fd9f15@arm.com>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?fnU59xuE9DDGa/mW6pr3NJq9Dk++I2xXCUoN+TcoXIeShRDtt1uqgmSQPSVj?=
 =?us-ascii?Q?MfemvOlIhr0Q6g+5GAKCxia0Y+xsniC8h1HpbukXjvZecDspMSBHJmis9VRF?=
 =?us-ascii?Q?gctDWURWnQNvceJYXUOJO2vPvEFnPIn4TIBQK5uWz+IEeoVZzhwBuDXoAdsW?=
 =?us-ascii?Q?gd1S0xP+9WqX8tyFL746x9Ab4sHx52ahraoBbbG/00LXjEhF79EKdNYeGnCW?=
 =?us-ascii?Q?YQcvuP+ctkSENetsGBmyTcxdzycYQRaU1PD/0of6BHlZPvfPaw4VcU7veTBU?=
 =?us-ascii?Q?a3+29fQZSE5IM27Wuy+CiePfTc1mnPo5LLUyCfqxax5P59tvOFQ/O5P/pREl?=
 =?us-ascii?Q?paNowCpq5IbGCB29mYEt5Ri4boaX/prS7X3fWxJi9qHnWuiGYN2OnJRw3ITs?=
 =?us-ascii?Q?7PKMFW56/mMvGBYEDfXr7JiCC950/U08VSRzwf9tadQqKRyQieqCXWZC9ZRj?=
 =?us-ascii?Q?TlZSCwARqgz3fyUurtdn+G38AFucg5EEHqhxAsXxL5ahaQODanbMr7YEE7Zr?=
 =?us-ascii?Q?7asTm7D1XuOLPrphlYMwcNt3DMgji2e43FkuoAE5E2A+2zKZNWNdV9syf02J?=
 =?us-ascii?Q?pOAgZ1Pe1vREDv8TinfkHjpYhGc0IgNYwce3xiaBYXr0PgW+/0xEi35fxKPH?=
 =?us-ascii?Q?c3KEUqs9PxnphdcLUoUakr6dGK/jfczHFwvKYSInfa/OmjgSasGTK7XJ9F2I?=
 =?us-ascii?Q?V31RdNrKTNvT9IaMtyD7w7xUhUX/jR4duSogj708rS4zCqdb0OE+sROg9Pte?=
 =?us-ascii?Q?dj+VjBQL16TXrF+zlm7BNVYthBieFaItt8C7l9tDPNKvoOw5WSk6kK6t/iW4?=
 =?us-ascii?Q?0xxUOlj+tWuDEo0fD/Xkntm97VnaYnocZdaT/RSdCgEnKLjqxrBf1zZiq/wK?=
 =?us-ascii?Q?CZrWVwE/slXS59cun31TOBX9ZcfEvgE39IIOU29mbJ29wOdA5kOXAXcSwZma?=
 =?us-ascii?Q?RZcbTahX2yICTUhtBNomDmMVir22ul5ViaW+IZrL6jufH7pfS1jOKf6cZSWA?=
 =?us-ascii?Q?L4+/DIHO6ep2HQ4x/uTCmAWwKDnfd8sH9F46ruhVOs74+A4kijArUjk2vBsD?=
 =?us-ascii?Q?GPnElJLxWNmi07Dm739B0cc+bk87Bv6+MA2/2EweITXhmH2bjJfP6Q9UO/ch?=
 =?us-ascii?Q?HS6uIdcBpLRZIMGgQ/W5Jm9ohvJE+xB5nlMXE+YLduAp667U/9B148dU+n+p?=
 =?us-ascii?Q?wfiEmYyZvAmTty1kg34hbS0BIScaXaBEj4rgkpnRy/L/n9h5a/OiHzQXkYaG?=
 =?us-ascii?Q?zY06xu4ReqoEF1Zzq7JrzB49PhmuJcDOe8/Eq9+7qw+aC/loUZStvBQ0n3wl?=
 =?us-ascii?Q?bggqEESEp8ZKzIQGcEV1fwsWbT+NkRERzvnwm3v7hHy4N92KnQ6nPC2jrOcu?=
 =?us-ascii?Q?NbMzNlcKwGZK35crYRhNWmaK+WBbpFeTvTbkCsyc0a1WuKMN3JrilhANtSZh?=
 =?us-ascii?Q?vbOIG+jNpRv0rNTYUFdB5pBFymy1aQ+25FhQ4Sf/AUMSVmD+vT1i1n2y9Qrw?=
 =?us-ascii?Q?gWt/Ov4pfl5ZqiqkmQHfTW9vWk4BYKW44x70Adt1DZySMooKuf5jFnv+xi/C?=
 =?us-ascii?Q?bEQKG8635jGsriGu2Zo=3D?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: d1a322e2-655c-4ce1-c702-08db70cdc89e
X-MS-Exchange-CrossTenant-AuthSource: LV2PR12MB5869.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Jun 2023 14:02:14.0524
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: uWRXjOVKbRQ4ZssnjeaO+YYLrWzCRmL8eCddTZrpgyoqh7zN7MXyS0D6d525rusr
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR12MB9078
X-Spam-Status: No, score=-1.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FORGED_SPF_HELO,
        RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_NONE,
        T_SCC_BODY_TEXT_LINE autolearn=no autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Jun 19, 2023 at 11:20:58AM +0100, Robin Murphy wrote:
> On 2023-06-16 19:59, Jason Gunthorpe wrote:
> > On Fri, Jun 16, 2023 at 05:34:53PM +0100, Robin Murphy wrote:
> > > 
> > > If the system has working ACS configured correctly, then this issue should
> > > be moot;
> > 
> > Yes
> > 
> > > if it doesn't, then a VFIO user is going to get a whole group of
> > > peer devices if they're getting anything at all, so it doesn't seem entirely
> > > unreasonable to leave it up to them to check that all those devices'
> > > resources play well with their expected memory map.
> > 
> > I think the kernel should be helping here.. 'go figure it out from
> > lspci' is a very convoluted and obscure uAPI, and I don't see things
> > like DPDK actually doing that.
> > 
> > IMHO the uAPI expectation is that the kernel informs userspace what
> > the usable IOVA is, if bridge windows and lack of ACS are rendering
> > address space unusable then VFIO/iommufd should return it as excluded
> > as well.
> > 
> > If we are going to do that then all UNAMANGED domain users should
> > follow the same logic.
> > 
> > We probably have avoided bug reports because of how rare it would be
> > to see a switch and an UNMANAGED domain using scenario together -
> > especially with ACS turned off.
> > 
> > So it is really narrow niche.. Obscure enough I'm not going to make
> > patches :)
> 
> The main thing is that we've already been round this once before; we tried
> it 6 years ago and then reverted it a year later for causing more problems
> than it solved:

As I said earlier in this thread if we do it for VFIO then the
calculation must be precise and consider bus details like
ACS/etc. eg VFIO on an ACS system should not report any new regions.

It looks like that thread confirms we can't create reserved regions
which are wrong :)

I think Alex is saying the same things I'm saying in that thread too:

https://lore.kernel.org/all/20180226161310.061ce3a8@w520.home/

(b) is what the kernel should help prevent.

And it is clear there are today scenarios where a VFIO user will get
data loss because the reported valid IOVA from the kernel is
incorrect. Fixing this is hard, much harder than what commit
273df9635385 ("iommu/dma: Make PCI window reservation generic") has.

Thanks,
Jason