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@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
---
Cc: Jan Beulich <jbeulich@suse.com>
---
Changes since v2:
 - Reword error message.

Changes since v1:
 - Reword commit message.
---
 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 d7a40d7550..15ed021f41 100644
--- a/tools/libs/light/libxl_create.c
+++ b/tools/libs/light/libxl_create.c
@@ -1160,17 +1160,17 @@ 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;
     }
 
-- 
2.34.1

On 17.02.2022 15:09, Roger Pau Monne wrote:
> 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@citrix.com>
> Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

I'm inclined to queue this for the stable trees. Thoughts?

Jan