[xen stable-4.16] tools/libxl: don't allow IOMMU usage with PoD

[patchbot@xxxxxxx]

commit 38fbfddf660de9572b1c906315a345b1470d3a04
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Mar 10 09:46:28 2022 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 10 09:46:28 2022 +0100

    tools/libxl: don't allow IOMMU usage with PoD
    
    Prevent libxl from creating guests that attempts to use PoD together
    with an IOMMU, even if no devices are actually assigned.
    
    While the hypervisor could support using PoD together with an IOMMU as
    long as no devices are assigned, such usage seems doubtful. There's no
    guarantee the guest has PoD no longer be active, and thus a later
    assignment of a PCI device to such domain could fail.
    
    Preventing the usage of PoD together with an IOMMU at guest creation
    avoids having to add checks for active PoD entries in the device
    assignment paths.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    master commit: 07449ecfa42532495156fa342af2112e3e31dd3f
    master date: 2022-02-18 09:03:08 +0100
---
 tools/libs/light/libxl_create.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/tools/libs/light/libxl_create.c b/tools/libs/light/libxl_create.c
index dcd09d32ba..2890697de8 100644
--- a/tools/libs/light/libxl_create.c
+++ b/tools/libs/light/libxl_create.c
@@ -1158,17 +1158,17 @@ int libxl__domain_config_setdefault(libxl__gc *gc,
     pod_enabled = (d_config->c_info.type != LIBXL_DOMAIN_TYPE_PV) &&
         (d_config->b_info.target_memkb < d_config->b_info.max_memkb);
 
-    /* We cannot have PoD and PCI device assignment at the same time
-     * for HVM guest. It was reported that IOMMU cannot work with PoD
-     * enabled because it needs to populated entire page table for
-     * guest. To stay on the safe side, we disable PCI device
-     * assignment when PoD is enabled.
+    /* We don't support having PoD and an IOMMU at the same time for HVM
+     * guests. An active IOMMU cannot work with PoD because it needs a fully
+     * populated page-table. Prevent PoD usage if the domain has an IOMMU
+     * assigned, even if not active.
      */
     if (d_config->c_info.type != LIBXL_DOMAIN_TYPE_PV &&
-        d_config->num_pcidevs && pod_enabled) {
+        d_config->c_info.passthrough != LIBXL_PASSTHROUGH_DISABLED &&
+        pod_enabled) {
         ret = ERROR_INVAL;
         LOGD(ERROR, domid,
-             "PCI device assignment for HVM guest failed due to PoD enabled");
+             "IOMMU required for device passthrough but not supported together 
with PoD");
         goto error_out;
     }
 
--
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