From nobody Wed Apr 8 10:14:29 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AC385C04A95 for ; Tue, 25 Oct 2022 08:08:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230419AbiJYIIU (ORCPT ); Tue, 25 Oct 2022 04:08:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59164 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229864AbiJYIIR (ORCPT ); Tue, 25 Oct 2022 04:08:17 -0400 Received: from mail.wantstofly.org (hmm.wantstofly.org [213.239.204.108]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1F2B27C1D7 for ; Tue, 25 Oct 2022 01:08:16 -0700 (PDT) Received: by mail.wantstofly.org (Postfix, from userid 1000) id A83C97F527; Tue, 25 Oct 2022 11:08:13 +0300 (EEST) Date: Tue, 25 Oct 2022 11:08:13 +0300 From: Lennert Buytenhek To: David Woodhouse , Lu Baolu Cc: Joerg Roedel , Will Deacon , Robin Murphy , iommu@lists.linux.dev, linux-kernel@vger.kernel.org Subject: [PATCH,RFC] iommu/vt-d: Convert dmar_fault IRQ to a threaded IRQ Message-ID: MIME-Version: 1.0 Content-Disposition: inline Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Under a high enough I/O page fault load, the dmar_fault hardirq handler can end up starving other tasks that wanted to run on the CPU that the IRQ is being routed to. On an i7-6700 CPU this seems to happen at around 2.5 million I/O page faults per second, and at a fraction of that rate on some of the lower-end CPUs that we use. An I/O page fault rate of 2.5 million per second may seem like a very high number, but when we get an I/O page fault for every cache line touched by a DMA operation, this I/O page fault rate can be the result of a confused PCIe device DMAing to RAM at 2.5 * 64 =3D 160 MB/sec, which is not an unlikely rate to be DMAing things to RAM at. And, in fact, when we do see PCIe devices getting confused like this, this sort of I/O page fault rate is not uncommon. A peripheral device continuously DMAing to RAM at 160 MB/s is inarguably a bug, either in the kernel driver for the device or in the firmware for the device, and should be fixed there, but it's the sort of bug that iommu/vt-d could be handling better than it currently does, and there is a fairly simple way to achieve that. This patch changes the dmar_fault IRQ handler to be a threaded IRQ handler. This is a pretty minimal code change, and comes with the advantage that Intel IOMMU I/O page fault handling work is now subject to RT throttling, which allows it to be kept under control using the sched_rt_period_us / sched_rt_runtime_us parameters. iommu/amd already uses a threaded IRQ handler for its I/O page fault reporting, and so it already has this advantage. When IRQ remapping is enabled, iommu/vt-d will try to set up its dmar_fault IRQ handler from start_kernel() -> x86_late_time_init() -> apic_intr_mode_init() -> apic_bsp_setup() -> irq_remap_enable_fault_handling() -> enable_drhd_fault_handling(), which happens before kthreadd is started, and trying to set up a threaded IRQ handler this early on will oops. However, there doesn't seem to be a reason why iommu/vt-d needs to set up its fault reporting IRQ handler this early, and if we remove the IRQ setup code from enable_drhd_fault_handling(), the IRQ will be registered instead from pci_iommu_init() -> intel_iommu_init() -> init_dmars(), which seems to work just fine. Suggested-by: Scarlett Gourley Suggested-by: James Sewart Suggested-by: Jack O'Sullivan Signed-off-by: Lennert Buytenhek --- drivers/iommu/intel/dmar.c | 27 ++------------------------- 1 file changed, 2 insertions(+), 25 deletions(-) diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c index 5a8f780e7ffd..d0871fe9d04d 100644 --- a/drivers/iommu/intel/dmar.c +++ b/drivers/iommu/intel/dmar.c @@ -2043,7 +2043,8 @@ int dmar_set_interrupt(struct intel_iommu *iommu) return -EINVAL; } =20 - ret =3D request_irq(irq, dmar_fault, IRQF_NO_THREAD, iommu->name, iommu); + ret =3D request_threaded_irq(irq, NULL, dmar_fault, IRQF_ONESHOT, + iommu->name, iommu); if (ret) pr_err("Can't request irq\n"); return ret; @@ -2051,30 +2052,6 @@ int dmar_set_interrupt(struct intel_iommu *iommu) =20 int __init enable_drhd_fault_handling(void) { - struct dmar_drhd_unit *drhd; - struct intel_iommu *iommu; - - /* - * Enable fault control interrupt. - */ - for_each_iommu(iommu, drhd) { - u32 fault_status; - int ret =3D dmar_set_interrupt(iommu); - - if (ret) { - pr_err("DRHD %Lx: failed to enable fault, interrupt, ret %d\n", - (unsigned long long)drhd->reg_base_addr, ret); - return -1; - } - - /* - * Clear any previous faults. - */ - dmar_fault(iommu->irq, iommu); - fault_status =3D readl(iommu->reg + DMAR_FSTS_REG); - writel(fault_status, iommu->reg + DMAR_FSTS_REG); - } - return 0; } =20 --=20 2.37.3