[PATCH for-4.20 2/3] x86/PCI: init segments earlier

[Jan Beulich <jbeulich@xxxxxxxx>]

In order for amd_iommu_detect_one_acpi()'s call to pci_ro_device() to
have permanent effect, pci_segments_init() needs to be called ahead of
making it there. Without this we're losing segment 0's r/o map, and thus
we're losing write-protection of the PCI devices representing IOMMUs.
Which in turn means that half-way recent Linux Dom0 will, as it boots,
turn off MSI on these devices, thus preventing any IOMMU events (faults
in particular) from being reported on pre-x2APIC hardware.

As the acpi_iommu_init() invocation was moved ahead of
acpi_mmcfg_init()'s by the offending commit, move the call to
pci_segments_init() accordingly.

Fixes: 3950f2485bbc ("x86/x2APIC: defer probe until after IOMMU ACPI table 
parsing")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Of course it would have been quite a bit easier to notice this issue if
radix_tree_insert() wouldn't work fine ahead of radix_tree_init() being
invoked for a given radix tree, when the index inserted at is 0.

While hunting down various other dead paths to actually find the root
cause, it occurred to me that it's probably not a good idea to fully
disallow config space writes for r/o devices: Dom0 won't be able to size
their BARs (luckily the IOMMU "devices" don't have any, but e.g. serial
ones generally will have at least one), for example. Without being able
to size BARs it also will likely be unable to correctly account for the
address space taken by these BARs. However, outside of vPCI it's not
really clear to me how we could reasonably emulate such BAR sizing
writes - we can't, after all, allow Dom0 to actually write to the
underlying physical registers, yet we don't intercept reads (i.e. we
can't mimic expected behavior then).

--- a/xen/arch/x86/x86_64/mmconfig-shared.c
+++ b/xen/arch/x86/x86_64/mmconfig-shared.c
@@ -402,8 +402,6 @@ void __init acpi_mmcfg_init(void)
 {
     bool valid = true;
 
-    pci_segments_init();
-
     /* MMCONFIG disabled */
     if ((pci_probe & PCI_PROBE_MMCONF) == 0)
         return;
--- a/xen/drivers/passthrough/x86/iommu.c
+++ b/xen/drivers/passthrough/x86/iommu.c
@@ -55,6 +55,8 @@ void __init acpi_iommu_init(void)
 {
     int ret = -ENODEV;
 
+    pci_segments_init();
+
     if ( !iommu_enable && !iommu_intremap )
         return;