[PATCH v1 1/3] xen/arm: Add a way to disable traps on unmapped MMIO

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

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

Add a per-domain way to optionally disable traps on unmapped MMIO.

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             |  2 ++
 xen/arch/arm/domain_build.c       |  3 +++
 xen/arch/arm/include/asm/domain.h |  2 ++
 xen/arch/arm/io.c                 | 33 +++++++++++++++++++++++++++++--
 xen/include/public/arch-arm.h     |  9 +++++++++
 7 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/tools/libs/light/libxl_arm.c b/tools/libs/light/libxl_arm.c
index 75c811053c..40cd005619 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 MMIO. */
+    config->arch.flags = XEN_ARM_TRAP_UNMAPPED_MMIO;
+
     return 0;
 }
 
diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
index a49764f0ad..e5e13e07d0 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 MMIO. */
+    d_cfg->arch.flags = XEN_ARM_TRAP_UNMAPPED_MMIO;
 }
 
 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..54c6ae7678 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -714,6 +714,8 @@ int arch_domain_create(struct domain *d,
     ioreq_domain_init(d);
 #endif
 
+    d->arch.trap_unmapped_mmio = config->arch.flags & 
XEN_ARM_TRAP_UNMAPPED_MMIO;
+
     /* p2m_init relies on some value initialized by the IOMMU subsystem */
     if ( (rc = iommu_domain_init(d, config->iommu_opts)) != 0 )
         goto fail;
diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index b189a7cfae..c3c8212260 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -2018,6 +2018,9 @@ void __init create_dom0(void)
     dom0_cfg.arch.tee_type = tee_get_type();
     dom0_cfg.max_vcpus = dom0_max_vcpus();
 
+    /* Dom0 always traps on unmapped MMIO.  */
+    dom0_cfg.arch.flags |= XEN_ARM_TRAP_UNMAPPED_MMIO;
+
     if ( iommu_enabled )
         dom0_cfg.flags |= XEN_DOMCTL_CDF_iommu;
 
diff --git a/xen/arch/arm/include/asm/domain.h 
b/xen/arch/arm/include/asm/domain.h
index a3487ca713..4d1a180ce2 100644
--- a/xen/arch/arm/include/asm/domain.h
+++ b/xen/arch/arm/include/asm/domain.h
@@ -121,6 +121,8 @@ struct arch_domain
     void *tee;
 #endif
 
+    bool trap_unmapped_mmio;
+
 }  __cacheline_aligned;
 
 struct arch_vcpu
diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index 5a4b0e8f25..11ffa48969 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)
@@ -178,8 +204,11 @@ enum io_state try_handle_mmio(struct cpu_user_regs *regs,
         rc = try_fwd_ioserv(regs, v, info);
         if ( rc == IO_HANDLED )
             return handle_ioserv(regs, v);
-
-        return rc;
+        else if ( rc == IO_UNHANDLED && !v->domain->arch.trap_unmapped_mmio ) {
+            /* Fallback to the unmapped handler. */
+            handler = &unmapped_handler;
+        } else
+            return rc;
     }
 
     /*
diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
index e2412a1747..32b023504d 100644
--- a/xen/include/public/arch-arm.h
+++ b/xen/include/public/arch-arm.h
@@ -350,6 +350,15 @@ struct xen_arch_domainconfig {
      *
      */
     uint32_t clock_frequency;
+    /*
+     * IN
+     *
+     * XEN_ARM_TRAP_UNMAPPED_MMIO enables trapping of memory accesses
+     * into unmapped ranges. When disabled, Xen will handle the access
+     * by reading 0xFFFFFFFF and ignoring writes.
+     */
+#define XEN_ARM_TRAP_UNMAPPED_MMIO (1U << 0)
+    uint32_t flags;
 };
 #endif /* __XEN__ || __XEN_TOOLS__ */
 
-- 
2.43.0