xen/riscv: introduce p2m_gpa_bits

common/device-tree/domain-build.c uses p2m_gpa_bits to determine the
upper bound of the GPA space when searching for unused regions, so it
must be defined when CONFIG_DOMAIN_BUILD_HELPERS=y.

The variable is initialised to PADDR_BITS and narrowed in p2m_init() to
the GPA width of the selected G-stage mode, allowing an external entity
(e.g. an IOMMU) to restrict it further if needed.

p2m_gpa_bits is a global rather than a per-domain value, which is
acceptable for now because all domains are required to use the same
G-stage MMU mode, as dom0less common code allocates it per all
domains.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/include/asm/p2m.h

@@ -47,6 +47,9 @@
 #define P2M_LEVEL_MASK(p2m, lvl) \
     (P2M_TABLE_OFFSET(p2m, lvl) << P2M_GFN_LEVEL_SHIFT(lvl))
 
+/* Holds the bit size of GPAs in p2m tables */
+extern unsigned int p2m_gpa_bits;
+
 #define paddr_bits PADDR_BITS
 
 /* Get host p2m table */

xen/arch/riscv/p2m.c

@@ -11,6 +11,8 @@
 #include <xen/sections.h>
 #include <xen/xvmalloc.h>
 
+#include <public/domctl.h>
+
 #include <asm/cpufeature.h>
 #include <asm/csr.h>
 #include <asm/flushtlb.h>
@@ -67,6 +69,12 @@ static const struct gstage_mode_desc gstage_modes[] = {
 const struct gstage_mode_desc *__ro_after_init max_gstage_mode =
     &gstage_modes[0];
 
+/*
+ * Set to the maximum configured support for GPA bits, so the number of GPA
+ * bits can be restricted by an external entity (e.g. IOMMU).
+ */
+unsigned int __ro_after_init p2m_gpa_bits = PADDR_BITS;
+
 static void p2m_free_page(struct p2m_domain *p2m, struct page_info *pg);
 
 static inline void p2m_free_metadata_page(struct p2m_domain *p2m,
@@ -355,6 +363,7 @@ int p2m_init(struct domain *d, const struct xen_domctl_createdomain *config)
      */
     static const struct gstage_mode_desc __ro_after_init *m = &gstage_modes[0];
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
+    unsigned int gpa_bits;
 
     /*
      * "Trivial" initialisation is now complete.  Set the backpointer so the
@@ -410,6 +419,12 @@ int p2m_init(struct domain *d, const struct xen_domctl_createdomain *config)
 #   error "Add init of p2m->clean_dcache"
 #endif
 
+    gpa_bits = P2M_GFN_LEVEL_SHIFT(p2m->mode->paging_levels + 1) +
+               P2M_ROOT_EXTRA_BITS;
+
+    if ( gpa_bits < p2m_gpa_bits )
+        p2m_gpa_bits = gpa_bits;
+
     return 0;
 }