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iommu/xen: reserve dom0's PCI windows, and fix the modular pcifront link #105

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Merged
kaniini merged 2 commits into from
Nov 14, 2026
+16 −3
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iommu/xen: reserve dom0's PCI windows, and fix the modular pcifront link #105

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55da034
iommu/xen: keep reserving the PCI host-bridge windows in dom0
kaniini Nov 13, 2026
9e5d4e3
xen/pcifront: do not promise pcifront_machine_sbdf() to built-in code…
kaniini Nov 13, 2026
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2 changes: 2 additions & 0 deletions drivers/iommu/Kconfig
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Original file line number Diff line number Diff line change
Expand Up @@ -366,6 +366,8 @@ config VIRTIO_IOMMU
config XEN_IOMMU
bool "Xen IOMMU driver"
depends on X86 && XEN
# Built-in, and asks pcifront for a device's machine BDF.
depends on XEN_PCIDEV_FRONTEND != m
select IOMMU_API
select IOMMU_IO_PGTABLE
help
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4 changes: 3 additions & 1 deletion drivers/iommu/dma-iommu.c
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Expand Up @@ -569,7 +569,9 @@ static int iova_reserve_iommu_regions(struct device *dev,
* are invented -- pcifront hands out iomem_resource itself, and a PVH
* guest with no host bridge _CRS gets Linux's catch-all default. There
* is nothing meaningful to reserve, and reserving it leaves no usable
* IOVA space whatsoever.
* IOVA space whatsoever. Dom0 sees the real host bridges and keeps the
* reservation: without it an IOVA can land on a sibling's BAR, and a
* fabric without ACS delivers the DMA there instead of to the IOMMU.
*/
if (dev_is_pci(dev) && !xen_iommu_manages_iova(dev)) {
ret = iova_reserve_pci_windows(to_pci_dev(dev), iovad);
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10 changes: 9 additions & 1 deletion drivers/iommu/xen-iommu.c
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Expand Up @@ -477,9 +477,17 @@ static struct iommu_ops xen_iommu_ops = {
},
};

/*
* True when the host-bridge windows @dev sees are not the machine's and
* dma-iommu must not reserve them: a guest's IOVA space belongs to the
* hypervisor, and the windows it sees are invented. Dom0 sees the real
* host bridges, whose windows route to real BARs, so there the reservation
* stands as on bare metal.
*/
bool xen_iommu_manages_iova(struct device *dev)
{
return dev->iommu && dev_iommu_ops(dev) == &xen_iommu_ops;
return !xen_initial_domain() && dev->iommu &&
dev_iommu_ops(dev) == &xen_iommu_ops;
}

static int __init xen_iommu_init(void)
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3 changes: 2 additions & 1 deletion include/xen/pci.h
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Expand Up @@ -8,7 +8,8 @@

struct pci_dev;

#if defined(CONFIG_XEN_PCIDEV_FRONTEND)
/* Called from built-in code, so a modular pcifront is as good as none. */
#if IS_BUILTIN(CONFIG_XEN_PCIDEV_FRONTEND)
int pcifront_machine_sbdf(const struct pci_dev *dev, u32 *sbdf);
#else
static inline int pcifront_machine_sbdf(const struct pci_dev *dev, u32 *sbdf)
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