Re: [PATCH v3 1/5] xen/arm: Add way to disable traps on accesses to unmapped addresses

[Julien Grall]

Hi Edgar,

On 30/05/2025 14:45, Edgar E. Iglesias wrote:
From: "Edgar E. Iglesias" <edgar.igles...@amd.com>
Add a per-domain way to optionally disable traps for accesses
to unmapped addresses.

The domain flag is general but it's only implemented for ARM for now.

Signed-off-by: Edgar E. Iglesias <edgar.igles...@amd.com>
---
  tools/libs/light/libxl_arm.c  |  3 +++
  xen/arch/arm/dom0less-build.c |  3 +++
  xen/arch/arm/domain.c         |  3 ++-
  xen/arch/arm/domain_build.c   |  3 ++-
  xen/arch/arm/io.c             | 37 +++++++++++++++++++++++++++++++++--
  xen/common/domain.c           |  3 ++-
  xen/include/public/domctl.h   |  4 +++-
  7 files changed, 50 insertions(+), 6 deletions(-)

diff --git a/tools/libs/light/libxl_arm.c b/tools/libs/light/libxl_arm.c
index 75c811053c..9530996e72 100644
--- a/tools/libs/light/libxl_arm.c
+++ b/tools/libs/light/libxl_arm.c
@@ -233,6 +233,9 @@ int libxl__arch_domain_prepare_config(libxl__gc *gc,
          config->arch.sve_vl = d_config->b_info.arch_arm.sve_vl / 128U;
      }
  
+    /* Trap accesses to unmapped areas. */
+    config->flags |= XEN_DOMCTL_CDF_trap_unmapped_accesses;
+
      return 0;
  }
  
diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
index a49764f0ad..a4e0a33632 100644
--- a/xen/arch/arm/dom0less-build.c
+++ b/xen/arch/arm/dom0less-build.c
@@ -343,6 +343,9 @@ void __init arch_create_domUs(struct dt_device_node *node,
          panic("'sve' property found, but CONFIG_ARM64_SVE not selected\n");
  #endif
      }
+
+    /* Trap accesses to unmapped areas. */
+    d_cfg->flags |= XEN_DOMCTL_CDF_trap_unmapped_accesses;
  }
  
  int __init init_intc_phandle(struct kernel_info *kinfo, const char *name,
diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 45aeb8bddc..be58a23dd7 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -612,7 +612,8 @@ int arch_sanitise_domain_config(struct 
xen_domctl_createdomain *config)
      unsigned int max_vcpus;
      unsigned int flags_required = (XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap);
      unsigned int flags_optional = (XEN_DOMCTL_CDF_iommu | XEN_DOMCTL_CDF_vpmu 
|
-                                   XEN_DOMCTL_CDF_xs_domain );
+                                   XEN_DOMCTL_CDF_xs_domain |
+                                   XEN_DOMCTL_CDF_trap_unmapped_accesses );

Just to double check, doesn't this mean the flag will be allowed on x86? 
If so, shouldn't we reject it in an arch?

Cheers,

--
Julien Grall