linux-kernel - [RFC v1 6/8] mshv: command line option to skip devices in PV-IOMMU

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Date:   Fri,  9 Jul 2021 11:43:37 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Linux on Hyper-V List <linux-hyperv@...r.kernel.org>
Cc:     virtualization@...ts.linux-foundation.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        kumarpraveen@...ux.microsoft.com, pasha.tatashin@...een.com,
        Wei Liu <wei.liu@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        iommu@...ts.linux-foundation.org (open list:IOMMU DRIVERS)
Subject: [RFC v1 6/8] mshv: command line option to skip devices in PV-IOMMU

Some devices may have been claimed by the hypervisor already. One such
example is a user can assign a NIC for debugging purpose.

Ideally Linux should be able to tell retrieve that information, but
there is no way to do that yet. And designing that new mechanism is
going to take time.

Provide a command line option for skipping devices. This is a stopgap
solution, so it is intentionally undocumented. Hopefully we can retire
it in the future.

Signed-off-by: Wei Liu <wei.liu@...nel.org>
---
 drivers/iommu/hyperv-iommu.c | 45 ++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/drivers/iommu/hyperv-iommu.c b/drivers/iommu/hyperv-iommu.c
index 043dcff06511..353da5036387 100644
--- a/drivers/iommu/hyperv-iommu.c
+++ b/drivers/iommu/hyperv-iommu.c
@@ -349,6 +349,16 @@ static const struct irq_domain_ops hyperv_root_ir_domain_ops = {
 
 #ifdef CONFIG_HYPERV_ROOT_PVIOMMU
 
+/* The IOMMU will not claim these PCI devices. */
+static char *pci_devs_to_skip;
+static int __init mshv_iommu_setup_skip(char *str) {
+	pci_devs_to_skip = str;
+
+	return 0;
+}
+/* mshv_iommu_skip=(SSSS:BB:DD.F)(SSSS:BB:DD.F) */
+__setup("mshv_iommu_skip=", mshv_iommu_setup_skip);
+
 /* DMA remapping support */
 struct hv_iommu_domain {
 	struct iommu_domain domain;
@@ -774,6 +784,41 @@ static struct iommu_device *hv_iommu_probe_device(struct device *dev)
 	if (!dev_is_pci(dev))
 		return ERR_PTR(-ENODEV);
 
+	/*
+	 * Skip the PCI device specified in `pci_devs_to_skip`. This is a
+	 * temporary solution until we figure out a way to extract information
+	 * from the hypervisor what devices it is already using.
+	 */
+	if (pci_devs_to_skip && *pci_devs_to_skip) {
+		int pos = 0;
+		int parsed;
+		int segment, bus, slot, func;
+		struct pci_dev *pdev = to_pci_dev(dev);
+
+		do {
+			parsed = 0;
+
+			sscanf(pci_devs_to_skip + pos,
+				" (%x:%x:%x.%x) %n",
+				&segment, &bus, &slot, &func, &parsed);
+
+			if (parsed <= 0)
+				break;
+
+			if (pci_domain_nr(pdev->bus) == segment &&
+				pdev->bus->number == bus &&
+				PCI_SLOT(pdev->devfn) == slot &&
+				PCI_FUNC(pdev->devfn) == func)
+			{
+				dev_info(dev, "skipped by MSHV IOMMU\n");
+				return ERR_PTR(-ENODEV);
+			}
+
+			pos += parsed;
+
+		} while (pci_devs_to_skip[pos]);
+	}
+
 	vdev = kzalloc(sizeof(*vdev), GFP_KERNEL);
 	if (!vdev)
 		return ERR_PTR(-ENOMEM);
-- 
2.30.2