[PATCHv2] x86/xen: during early setup, only 1:1 map the ISA region

[David Vrabel <david.vrabel@citrix.com>]

From: David Vrabel <david.vrabel@citrix.com>

During early setup, when the reserved regions and MMIO holes are being
setup as 1:1 in the p2m, clear any mappings instead of making them 1:1
(execept for the ISA region which is expected to be mapped).

This fixes a regression introduced in 3.5 by 83d51ab473dd (xen/setup:
update VA mapping when releasing memory during setup) which caused
hosts with tboot to fail to boot.

tboot marks a region in the e820 map as unusable and the dom0 kernel
would attempt to map this region and Xen does not permit unusable
regions to be mapped by guests.

(XEN)  0000000000000000 - 0000000000060000 (usable)
(XEN)  0000000000060000 - 0000000000068000 (reserved)
(XEN)  0000000000068000 - 000000000009e000 (usable)
(XEN)  0000000000100000 - 0000000000800000 (usable)
(XEN)  0000000000800000 - 0000000000972000 (unusable)

tboot marked this region as unusable.

(XEN)  0000000000972000 - 00000000cf200000 (usable)
(XEN)  00000000cf200000 - 00000000cf38f000 (reserved)
(XEN)  00000000cf38f000 - 00000000cf3ce000 (ACPI data)
(XEN)  00000000cf3ce000 - 00000000d0000000 (reserved)
(XEN)  00000000e0000000 - 00000000f0000000 (reserved)
(XEN)  00000000fe000000 - 0000000100000000 (reserved)
(XEN)  0000000100000000 - 0000000630000000 (usable)

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
v2: Extend 1:1 mapping region to cover 0 - 1MiB. find_ibft_region()
scans from 512 KiB and if this overlapped with a reserved region it
would crash.
---
 arch/x86/xen/setup.c |   16 +++++++++++-----
 1 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 94eac5c..9411756 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -215,13 +215,19 @@ static void __init xen_set_identity_and_release_chunk(
 	unsigned long pfn;
 
 	/*
-	 * If the PFNs are currently mapped, the VA mapping also needs
-	 * to be updated to be 1:1.
+	 * If the PFNs are currently mapped, clear the mappings
+	 * (except for the ISA region which must be 1:1 mapped) to
+	 * release the refcounts (in Xen) on the original frames.
 	 */
-	for (pfn = start_pfn; pfn <= max_pfn_mapped && pfn < end_pfn; pfn++)
+	for (pfn = start_pfn; pfn <= max_pfn_mapped && pfn < end_pfn; pfn++) {
+		pte_t pte = __pte_ma(0);
+
+		if (pfn < PFN_UP(ISA_END_ADDRESS))
+			pte = mfn_pte(pfn, PAGE_KERNEL_IO);
+
 		(void)HYPERVISOR_update_va_mapping(
-			(unsigned long)__va(pfn << PAGE_SHIFT),
-			mfn_pte(pfn, PAGE_KERNEL_IO), 0);
+			(unsigned long)__va(pfn << PAGE_SHIFT), pte, 0);
+	}
 
 	if (start_pfn < nr_pages)
 		*released += xen_release_chunk(
-- 
1.7.2.5