docs/misra: add R21.6 R21.14 R21.15 R21.16

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Also add two specific project-wide deviations for R21.6 and R21.15.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>

diff --git a/docs/misra/deviations.rst b/docs/misra/deviations.rst
index 32b02905d1..9123c8edb5 100644
--- a/docs/misra/deviations.rst
+++ b/docs/misra/deviations.rst
@@ -387,6 +387,22 @@ Deviations related to MISRA C:2012 Rules:
        of the Rule due to uses of this macro.
      - Tagged as `deliberate` for ECLAIR.
 
+   * - R21.6
+     - The use of snprintf() and vsnprintf() is justifiable as, despite
+       the fact that such functions have the same names of the
+       corresponding standard library functions, each configuration of
+       Xen has a unique implementation for them; the code implementing
+       such functions is subject to the analysis, so that any undefined
+       or unspecified behavior associated to them falls under the
+       responsibility of other MISRA guidelines
+     - Tagged as `safe` for ECLAIR.
+
+   * - R21.15
+     - The use of void* arguments is justifiable as the rationale for
+       the rule is to indicate possible mistakes, and void* is
+       frequently used in Xen to represent virtual memory addresses
+     - Tagged as `deliberate` for ECLAIR.
+
 Other deviations:
 -----------------
 
diff --git a/docs/misra/rules.rst b/docs/misra/rules.rst
index b7b447e152..00db02ad34 100644
--- a/docs/misra/rules.rst
+++ b/docs/misra/rules.rst
@@ -652,12 +652,38 @@ maintainers if you want to suggest a change.
        declared
      - See comment for Rule 21.1
 
+   * - `Rule 21.6 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_06.c>`_
+     - Required
+     - The Standard Library input/output routines shall not be used
+     - See the snprintf() and vsnprintf() deviation in deviations.rst
+
    * - `Rule 21.13 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_13.c>`_
      - Mandatory
      - Any value passed to a function in <ctype.h> shall be representable as an
        unsigned char or be the value EOF
      -
 
+   * - `Rule 21.14 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_14.c>`_
+     - Required
+     - The Standard Library function memcmp shall not be used to compare
+       null terminated strings
+     -
+
+   * - `Rule 21.15 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_15.c>`_
+     - Required
+     - The pointer arguments to the Standard Library functions memcpy,
+       memmove and memcmp shall be pointers to qualified or unqualified
+       versions of compatible types 
+     - void* arguments are allowed, see deviations.rst
+
+   * - `Rule 21.16 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_16.c>`_
+     - Required
+     - The pointer arguments to the Standard Library function memcmp
+       shall point to either a pointer type, an essentially signed type,
+       an essentially unsigned type, an essentially Boolean type or an
+       essentially enum type 
+     -
+
    * - `Rule 21.17 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_17.c>`_
      - Mandatory
      - Use of the string handling functions from <string.h> shall not result in