[PATCH v5 1/4] xen/riscv: add VM space layout

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Also it was added explanation about ignoring of top VA bits

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in V5:
* the patch was introduced in the current patch series.
---
 xen/arch/riscv/include/asm/config.h | 31 +++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/xen/arch/riscv/include/asm/config.h 
b/xen/arch/riscv/include/asm/config.h
index 763a922a04..0c860e88ce 100644
--- a/xen/arch/riscv/include/asm/config.h
+++ b/xen/arch/riscv/include/asm/config.h
@@ -4,6 +4,37 @@
 #include <xen/const.h>
 #include <xen/page-size.h>
 
+/*
+ * RISC-V64 Layout:
+ *
+ * From the riscv-privileged doc:
+ *   When mapping between narrower and wider addresses,
+ *   RISC-V zero-extends a narrower physical address to a wider size.
+ *   The mapping between 64-bit virtual addresses and the 39-bit usable
+ *   address space of Sv39 is not based on zero-extension but instead
+ *   follows an entrenched convention that allows an OS to use one or
+ *   a few of the most-significant bits of a full-size (64-bit) virtual
+ *   address to quickly distinguish user and supervisor address regions.
+ *
+ * It means that:
+ *   top VA bits are simply ignored for the purpose of translating to PA.
+ *
+ * The similar is true for other Sv{32, 39, 48, 57}.
+ *
+ * ============================================================================
+ *    Start addr    |   End addr        |  Size  | VM area description
+ * ============================================================================
+ * FFFFFFFFC0000000 |  FFFFFFFFC0200000 |  2 MB  | Xen
+ * FFFFFFFFC0200000 |  FFFFFFFFC0600000 |  4 MB  | FDT
+ * FFFFFFFFC0600000 |  FFFFFFFFC0800000 |  2 MB  | Fixmap
+ *     .................. unused ..................
+ * 0000003200000000 |  0000007f40000000 | 331 GB | Direct map(L2 slot: 200-509)
+ * 0000003100000000 |  0000003140000000 |  1 GB  | Frametable(L2 slot: 196-197)
+ * 0000003080000000 |  00000030c0000000 |  1 GB  | VMAP (L2 slot: 194-195)
+ *     .................. unused ..................
+ * ============================================================================
+ */
+
 #if defined(CONFIG_RISCV_64)
 # define LONG_BYTEORDER 3
 # define ELFSIZE 64
-- 
2.39.2