[PATCH v2 1/3] xen/arm: Add way to disable traps on accesses to unmapped addresses

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxx>]

From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>

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.iglesias@xxxxxxx>
---
 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             | 36 +++++++++++++++++++++++++++++++++--
 xen/common/domain.c           |  3 ++-
 xen/include/public/domctl.h   |  4 +++-
 7 files changed, 49 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 );
     unsigned int sve_vl_bits = sve_decode_vl(config->arch.sve_vl);
 
     if ( (config->flags & ~flags_optional) != flags_required )
diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index b189a7cfae..7ff9c1b584 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -2003,7 +2003,8 @@ void __init create_dom0(void)
 {
     struct domain *dom0;
     struct xen_domctl_createdomain dom0_cfg = {
-        .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap,
+        .flags = XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap |
+                 XEN_DOMCTL_CDF_trap_unmapped_accesses,
         .max_evtchn_port = -1,
         .max_grant_frames = gnttab_dom0_frames(),
         .max_maptrack_frames = -1,
diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index 5a4b0e8f25..adfc822e7e 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -21,6 +21,32 @@
 
 #include "decode.h"
 
+/* Handler for unmapped ranges. Writes ignored, reads return all ones.  */
+static int unmapped_read(struct vcpu *v, mmio_info_t *info, register_t *r,
+                         void *priv)
+{
+    uint64_t mask = GENMASK_ULL((1U << info->dabt.size) * 8 - 1, 0);
+
+    /* Mask off upper bits.  */
+    *r = UINT64_MAX & mask;
+    return 1;
+}
+
+static int unmapped_write(struct vcpu *v, mmio_info_t *info, register_t r,
+                          void *priv)
+{
+    return 1;
+}
+
+static const struct mmio_handler_ops unmapped_ops = {
+    .read = unmapped_read,
+    .write = unmapped_write
+};
+
+static const struct mmio_handler unmapped_handler = {
+    .ops = &unmapped_ops
+};
+
 static enum io_state handle_read(const struct mmio_handler *handler,
                                  struct vcpu *v,
                                  mmio_info_t *info)
@@ -175,11 +201,17 @@ enum io_state try_handle_mmio(struct cpu_user_regs *regs,
     handler = find_mmio_handler(v->domain, info->gpa);
     if ( !handler )
     {
+        bool trap_unmapped = v->domain->options &
+                                         XEN_DOMCTL_CDF_trap_unmapped_accesses;
         rc = try_fwd_ioserv(regs, v, info);
         if ( rc == IO_HANDLED )
             return handle_ioserv(regs, v);
-
-        return rc;
+        else if ( rc == IO_UNHANDLED && !trap_unmapped )
+        {
+            /* Fallback to the unmapped handler. */
+            handler = &unmapped_handler;
+        } else
+            return rc;
     }
 
     /*
diff --git a/xen/common/domain.c b/xen/common/domain.c
index abf1969e60..ac4f58f638 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -721,7 +721,8 @@ static int sanitise_domain_config(struct 
xen_domctl_createdomain *config)
          ~(XEN_DOMCTL_CDF_hvm | XEN_DOMCTL_CDF_hap |
            XEN_DOMCTL_CDF_s3_integrity | XEN_DOMCTL_CDF_oos_off |
            XEN_DOMCTL_CDF_xs_domain | XEN_DOMCTL_CDF_iommu |
-           XEN_DOMCTL_CDF_nested_virt | XEN_DOMCTL_CDF_vpmu) )
+           XEN_DOMCTL_CDF_nested_virt | XEN_DOMCTL_CDF_vpmu |
+           XEN_DOMCTL_CDF_trap_unmapped_accesses) )
     {
         dprintk(XENLOG_INFO, "Unknown CDF flags %#x\n", config->flags);
         return -EINVAL;
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 5b2063eed9..be19ab5e26 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -66,9 +66,11 @@ struct xen_domctl_createdomain {
 #define XEN_DOMCTL_CDF_nested_virt    (1U << _XEN_DOMCTL_CDF_nested_virt)
 /* Should we expose the vPMU to the guest? */
 #define XEN_DOMCTL_CDF_vpmu           (1U << 7)
+/* Should we trap guest accesses to unmapped addresses? */
+#define XEN_DOMCTL_CDF_trap_unmapped_accesses  (1U << 8)
 
 /* Max XEN_DOMCTL_CDF_* constant.  Used for ABI checking. */
-#define XEN_DOMCTL_CDF_MAX XEN_DOMCTL_CDF_vpmu
+#define XEN_DOMCTL_CDF_MAX XEN_DOMCTL_CDF_trap_unmapped_accesses
 
     uint32_t flags;
 
-- 
2.43.0