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Wed, 7 Aug 2024 10:18:20 -0500 From: Stewart Hildebrand To: Bjorn Helgaas , =?utf-8?q?Ilpo_J=C3=A4rvinen?= CC: Stewart Hildebrand , , Subject: [PATCH v3 8/8] PCI: Align small BARs Date: Wed, 7 Aug 2024 11:17:17 -0400 Message-ID: <20240807151723.613742-9-stewart.hildebrand@amd.com> X-Mailer: git-send-email 2.46.0 In-Reply-To: <20240807151723.613742-1-stewart.hildebrand@amd.com> References: <20240807151723.613742-1-stewart.hildebrand@amd.com> Precedence: bulk X-Mailing-List: linux-pci@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Received-SPF: None (SATLEXMB03.amd.com: stewart.hildebrand@amd.com does not designate permitted sender hosts) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS2PEPF0000343A:EE_|IA0PR12MB8421:EE_ X-MS-Office365-Filtering-Correlation-Id: 2c9e4a61-4369-46eb-1628-08dcb6f42d09 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|376014|1800799024|36860700013; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Aug 2024 15:18:22.2519 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 2c9e4a61-4369-46eb-1628-08dcb6f42d09 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[SATLEXMB03.amd.com] X-MS-Exchange-CrossTenant-AuthSource: DS2PEPF0000343A.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PR12MB8421 In this context, "small" is defined as less than max(SZ_4K, PAGE_SIZE). Issues observed when small BARs are not sufficiently aligned are: 1. Devices to be passed through (to e.g. a Xen HVM guest) with small BARs require each memory BAR to be page aligned. Currently, the only way to guarantee this alignment from a user perspective is to fake the size of the BARs using the pci=resource_alignment= option. This is a bad user experience, and faking the BAR size is not always desirable. For example, pcitest is a tool that is useful for PCI passthrough validation with Xen, but pcitest fails with a fake BAR size. 2. Devices with multiple small BARs could have the MSI-X tables located in one of its small BARs. This may lead to the MSI-X tables being mapped in the same 4k region as other data. The PCIe 6.1 specification (section 7.7.2 MSI-X Capability and Table Structure) says we probably should avoid that. To improve the user experience (i.e. don't require the user to specify pci=resource_alignment=), and increase conformance to PCIe spec, set the default minimum resource alignment of memory BARs to the greater of 4k or PAGE_SIZE. Quoting the comment in drivers/pci/pci.c:pci_request_resource_alignment(), there are two ways we can increase the resource alignment: 1) Increase the size of the resource. BARs are aligned on their size, so when we reallocate space for this resource, we'll allocate it with the larger alignment. This also prevents assignment of any other BARs inside the alignment region, so if we're requesting page alignment, this means no other BARs will share the page. The disadvantage is that this makes the resource larger than the hardware BAR, which may break drivers that compute things based on the resource size, e.g., to find registers at a fixed offset before the end of the BAR. 2) Retain the resource size, but use IORESOURCE_STARTALIGN and set r->start to the desired alignment. By itself this doesn't prevent other BARs being put inside the alignment region, but if we realign *every* resource of every device in the system, none of them will share an alignment region. Changing pcibios_default_alignment() results in the second method of alignment with IORESOURCE_STARTALIGN. The new default alignment may be overridden by arches by implementing pcibios_default_alignment(), or by the user on a per-device basis with the pci=resource_alignment= option (although this reverts to using IORESOURCE_SIZEALIGN). Signed-off-by: Stewart Hildebrand --- Preparatory patches in this series are prerequisites to this patch. v2->v3: * new subject (was: "PCI: Align small (<4k) BARs") * clarify 4k vs PAGE_SIZE in commit message v1->v2: * capitalize subject text * s/4 * 1024/SZ_4K/ * #include * update commit message * use max(SZ_4K, PAGE_SIZE) for alignment value --- drivers/pci/pci.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c index af34407f2fb9..efdd5b85ea8c 100644 --- a/drivers/pci/pci.c +++ b/drivers/pci/pci.c @@ -31,6 +31,7 @@ #include #include #include +#include #include "pci.h" DEFINE_MUTEX(pci_slot_mutex); @@ -6484,7 +6485,12 @@ struct pci_dev __weak *pci_real_dma_dev(struct pci_dev *dev) resource_size_t __weak pcibios_default_alignment(void) { - return 0; + /* + * Avoid MSI-X tables being mapped in the same 4k region as other data + * according to PCIe 6.1 specification section 7.7.2 MSI-X Capability + * and Table Structure. + */ + return max(SZ_4K, PAGE_SIZE); } /*