[xen master] VT-d: refuse to use IOMMU with reserved CAP.ND value

[patchbot@xxxxxxx]

commit a1545fbf45c689aff39ce76a6eaa609d32ef72a7
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Apr 20 10:54:26 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Apr 20 10:54:26 2022 +0200

    VT-d: refuse to use IOMMU with reserved CAP.ND value
    
    The field taking the value 7 (resulting in 18-bit DIDs when using the
    calculation in cap_ndoms(), when the DID fields are only 16 bits wide)
    is reserved. Instead of misbehaving in case we would encounter such an
    IOMMU, refuse to use it.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 xen/drivers/passthrough/vtd/iommu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/iommu.c 
b/xen/drivers/passthrough/vtd/iommu.c
index 1e309dedf4..657e805987 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -1267,8 +1267,11 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
 
     quirk_iommu_caps(iommu);
 
+    nr_dom = cap_ndoms(iommu->cap);
+
     if ( cap_fault_reg_offset(iommu->cap) +
          cap_num_fault_regs(iommu->cap) * PRIMARY_FAULT_REG_LEN > PAGE_SIZE ||
+         ((nr_dom - 1) >> 16) /* I.e. cap.nd > 6 */ ||
          (has_register_based_invalidation(iommu) &&
           ecap_iotlb_offset(iommu->ecap) >= PAGE_SIZE) )
     {
@@ -1294,8 +1297,6 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
     if ( !ecap_coherent(iommu->ecap) )
         iommu_non_coherent = true;
 
-    nr_dom = cap_ndoms(iommu->cap);
-
     if ( nr_dom <= DOMID_MASK * 2 + cap_caching_mode(iommu->cap) )
     {
         /* Allocate domain id (bit) maps. */
--
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