docs: Improve documentation and parsing for efi=

Update parse_efi_param() to use parse_boolean() for "rs", so it behaves
like other Xen booleans.

However, change "attr=uc" to not be a boolean.  "no-attr=uc" is ambiguous and
shouldn't be accepted, but accept "attr=no" as an acceptable alternative.

Update the command line documentation for consistency.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

docs/misc/xen-command-line.pandoc

@@ -853,23 +853,18 @@ disable it (edid=no). This option should not normally be required
 except for debugging purposes.
 
 ### efi
-> `= List of [ rs | attr ]`
+    = List of [ rs=<bool>, attr=no|uc ]
 
-All options are of boolean kind and can be prefixed with `no-` to
-effect the inverse meaning.
+Controls for interacting with the system Extended Firmware Interface.
 
-> `rs`
+*   The `rs` boolean controls whether Runtime Services are used.  By default,
+    Xen uses Runtime Services itself, and proxies certain calls on behalf of
+    dom0.  Selecting `rs=0` prohibits all use of Runtime Services.
 
-> Default: `true`
-
->> Force or disable use of EFI runtime services.
-
-> `attr=uc`
-
-> Default: `off`
-
->> Allows mapping of RuntimeServices which have no cachability attribute
->> set as UC.
+*   The `attr=` string exists to specify what to do with memory regions of
+    unknown/unrecognised cacheability.  `attr=no` is the default and will
+    leave the memory regions unmapped, while `attr=uc` will map them as fully
+    uncacheable.
 
 ### ept
 > `= List of [ ad=<bool>, pml=<bool> ]`

xen/common/efi/boot.c

@@ -1389,27 +1389,29 @@ static bool __initdata efi_map_uc;
 static int __init parse_efi_param(const char *s)
 {
     const char *ss;
-    int rc = 0;
+    int rc = 0, val;
 
     do {
-        bool val = strncmp(s, "no-", 3);
-
-        if ( !val )
-            s += 3;
-
         ss = strchr(s, ',');
         if ( !ss )
             ss = strchr(s, '\0');
 
-        if ( !cmdline_strcmp(s, "rs") )
+        if ( (val = parse_boolean("rs", s, ss)) >= 0 )
         {
             if ( val )
                 __set_bit(EFI_RS, &efi_flags);
             else
                 __clear_bit(EFI_RS, &efi_flags);
         }
-        else if ( !cmdline_strcmp(s, "attr=uc") )
-            efi_map_uc = val;
+        else if ( (ss - s) > 5 && !memcmp(s, "attr=", 5) )
+        {
+            if ( cmdline_strcmp(s + 5, "uc") )
+                efi_map_uc = true;
+            else if ( cmdline_strcmp(s + 5, "no") )
+                efi_map_uc = false;
+            else
+                rc = -EINVAL;
+        }
         else
             rc = -EINVAL;