Add lesson 60 readme

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>

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+Before we start creating HII forms that would work with non-volatile UEFI variables (i.e. persistent across reboots), we need to check UEFI variable services one more time.
+
+Up until now we've only checked already existing variables with the help of `gRT->GetVariable` and `gRT->GetNextVariableName` services.
+
+Let's try to create an application that would create, change and delete our own custom UEFI variables.
+
+For this task we will need `gRT->SetVariable` function:
+```
+SetVariable()
+
+Summary:
+Sets the value of a variable.
+
+Prototype:
+typedef
+EFI_STATUS
+SetVariable (
+ IN CHAR16 *VariableName,
+ IN EFI_GUID *VendorGuid,
+ IN UINT32 Attributes,
+ IN UINTN DataSize,
+ IN VOID *Data
+ );
+
+Parameters:
+VariableName 	A Null-terminated string that is the name of the vendor’s variable.
+		Each VariableName is unique for each VendorGuid.
+		VariableName must contain 1 or more characters. If VariableName is an empty string, then
+		EFI_INVALID_PARAMETER is returned
+VendorGuid 	A unique identifier for the vendor
+Attributes 	Attributes bitmask to set for the variable
+DataSize 	The size in bytes of the Data buffer ... A size of zero causes the variable to be deleted.
+Data 		The contents for the variable
+```
+
+Now let's write a code for our application. It would give a user a possibility to create or delete custom UEFI variables. For the simplicity the variable value would be just a string supplied by user. Here is a help message for our application:
+
+```
+VOID Usage()
+{
+  Print(L"Delete variable\n");
+  Print(L"   SetVariableExample <variable name>\n");
+  Print(L"\n");
+  Print(L"Set variable\n");
+  Print(L"   SetVariableExample <variable name> <attributes> <value>\n");
+  Print(L"\n");
+  Print(L"<attributes> can be <n|b|r>\n");
+  Print(L"n - NON_VOLATILE\n");
+  Print(L"b - BOOTSERVICE_ACCESS\n");
+  Print(L"r - RUNTIME_ACCESS\n");
+}
+```
+
+You can see that when variable is created it is necessary to provide variable attributes. They can be a combination of the following flags:
+- `EFI_VARIABLE_NON_VOLATILE` - variable is persistent across reboots
+- `EFI_VARIABLE_BOOTSERVICE_ACCESS` - variable is available in the UEFI stage
+- `EFI_VARIABLE_RUNTIME_ACCESS` - variable is available in the OS stage (after the successfull `EFI_BOOT_SERVICES.ExitBootServices()` call)
+
+These flags are defined in the https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Uefi/UefiMultiPhase.h with the several other flags that we wouldn't cover right now.
+
+With these 3 flags only the following combinations are allowed:
+
+| NV | BS | RT |
+|---|---|---|
+| + | + | + |
+| + | + |   |
+|   | + |   |
+|   | + | + |
+
+
+Sometimes in the code you could encounter variable attribute combinations like these (https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Include/Guid/VariableFormat.h):
+```
+///
+/// Variable Attribute combinations.
+///
+#define VARIABLE_ATTRIBUTE_NV_BS           (EFI_VARIABLE_NON_VOLATILE | EFI_VARIABLE_BOOTSERVICE_ACCESS)
+#define VARIABLE_ATTRIBUTE_BS_RT           (EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS)
+#define VARIABLE_ATTRIBUTE_NV_BS_RT        (VARIABLE_ATTRIBUTE_BS_RT | EFI_VARIABLE_NON_VOLATILE)
+...
+```
+
+But let's get back to our application. We would need to handle command arguments so it is better to create a shell application.
+
+The code is pretty simple.
+
+`UefiLessonsPkg/SetVariableExample/SetVariableExample.c`
+```
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiRuntimeServicesTableLib.h>
+#include <Library/UefiLib.h>
+
+
+VOID Usage()
+{
+  ...
+}
+
+INTN EFIAPI ShellAppMain(IN UINTN Argc, IN CHAR16 **Argv)
+{
+  EFI_STATUS Status;
+
+  if (Argc==2) {
+    CHAR16* VariableName = Argv[1];
+    Status = gRT->SetVariable (
+                VariableName,
+                &gEfiCallerIdGuid,
+                0,
+                0,
+                NULL
+                );
+    if (EFI_ERROR(Status)) {
+      Print(L"%r\n", Status);
+    } else {
+      Print(L"Variable %s was successfully deleted\n", VariableName);
+    }
+    return Status;
+  } else if (Argc==4) {
+    CHAR16* VariableName = Argv[1];
+    CHAR16* AttributesStr = Argv[2];
+    CHAR16* Value = Argv[3];
+    UINT32  Attributes = 0;
+    for (UINTN i=0; i<StrLen(AttributesStr); i++) {
+      switch(AttributesStr[i]) {
+        case L'n':
+          Attributes |= EFI_VARIABLE_NON_VOLATILE;
+          break;
+        case L'b':
+          Attributes |= EFI_VARIABLE_BOOTSERVICE_ACCESS;
+          break;
+        case L'r':
+          Attributes |= EFI_VARIABLE_RUNTIME_ACCESS;
+          break;
+        default:
+          Print(L"Error! Unknown attribute!");
+          return EFI_INVALID_PARAMETER;
+      }
+    }
+    Status = gRT->SetVariable (
+                VariableName,
+                &gEfiCallerIdGuid,
+                Attributes,
+                StrSize(Value),
+                Value
+                );
+    if (EFI_ERROR(Status)) {
+      Print(L"%r\n", Status);
+    } else {
+      Print(L"Variable %s was successfully changed\n", VariableName);
+    }
+    return Status;
+  } else {
+    Usage();
+  }
+
+  return EFI_SUCCESS;
+}
+```
+`UefiLessonsPkg/SetVariableExample/SetVariableExample.inf`:
+```
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = SetVariableExample
+  FILE_GUID                      = bb2a829f-7943-4691-a03a-f1f48519d7e6
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = ShellCEntryLib
+
+[Sources]
+  SetVariableExample.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
+  UefiLib
+  ShellCEntryLib
+```
+
+When `Argc==4`, we treat incoming arguments as inputs for the variable creation. We parse arguments and call `gRT->SetVariable` service.
+
+When `Argc==2`, it means that the user have supplied only the variable name and according to our help message we have to delete this variable. To do it we call `gRT->SetVariable` with a `DataSize` equal to 0.
+
+All the variables would be created under the `gEfiCallerIdGuid` GUID, which in our case means `bb2a829f-7943-4691-a03a-f1f48519d7e6`.
+
+Build our application and copy it to the UEFI shared disk folder.
+
+Don't forget to add `-drive if=pflash,format=raw,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF_VARS.fd` to the QEMU arguments:
+```
+qemu-system-x86_64 \
+  -drive if=pflash,format=raw,readonly,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF_CODE.fd \
+  -drive if=pflash,format=raw,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF_VARS.fd \
+  -drive format=raw,file=fat:rw:~/UEFI_disk \
+  -net none \
+  -nographic
+```
+
+To look at the currently present UEFI variables we can use `dmpstore` command from the UEFI shell. This command has an option to print only variables associated with a particular guid. If we try to execute it right from the UEFI shell start we wouldn't get anything associated with our GUID:
+```
+FS0:\> dmpstore -guid bb2a829f-7943-4691-a03a-f1f48519d7e6
+dmpstore: No matching variables found. Guid BB2A829F-7943-4691-A03A-F1F48519D7E6
+```
+
+Let's try to create a `MyVar` variable with a `Hello` content:
+```
+FS0:\> SetVariableExample.efi MyVar b "Hello"
+Variable MyVar was successfully changed
+FS0:\> dmpstore -guid bb2a829f-7943-4691-a03a-f1f48519d7e6
+Variable BS 'BB2A829F-7943-4691-A03A-F1F48519D7E6:MyVar' DataSize = 0x0C
+  00000000: 48 00 65 00 6C 00 6C 00-6F 00 00 00              *H.e.l.l.o...*
+```
+You can indeed see that the variable was created.
+
+You can try to change it:
+```
+FS0:\> SetVariableExample.efi MyVar b "Hello World"
+Variable MyVar was successfully changed
+FS0:\> dmpstore -guid bb2a829f-7943-4691-a03a-f1f48519d7e6
+Variable BS 'BB2A829F-7943-4691-A03A-F1F48519D7E6:MyVar' DataSize = 0x18
+  00000000: 48 00 65 00 6C 00 6C 00-6F 00 20 00 57 00 6F 00  *H.e.l.l.o. .W.o.*
+  00000010: 72 00 6C 00 64 00 00 00-                         *r.l.d...*
+```
+Or delete it:
+```
+FS0:\> SetVariableExample.efi MyVar
+Variable MyVar was successfully deleted
+FS0:\> dmpstore -guid bb2a829f-7943-4691-a03a-f1f48519d7e6
+dmpstore: No matching variables found. Guid BB2A829F-7943-4691-A03A-F1F48519D7E6
+```
+
+You can see the proof that not all the combination of attributes are supported:
+```
+FS0:\> SetVariableExample.efi MyVar nr "Hello"
+Invalid Parameter
+```
+
+
+Now let's create two variables with the different attributes: "NV+BS" and "BS"
+```
+FS0:\> SetVariableExample.efi MyPersistentVar nb "Persistent variable"
+Variable MyPersistentVar was successfully changed
+FS0:\> SetVariableExample.efi MyVar b "Memory variable"
+Variable MyVar was successfully changed
+FS0:\> dmpstore -guid bb2a829f-7943-4691-a03a-f1f48519d7e6
+Variable NV+BS 'BB2A829F-7943-4691-A03A-F1F48519D7E6:MyPersistentVar' DataSize = 0x28
+  00000000: 50 00 65 00 72 00 73 00-69 00 73 00 74 00 65 00  *P.e.r.s.i.s.t.e.*
+  00000010: 6E 00 74 00 20 00 76 00-61 00 72 00 69 00 61 00  *n.t. .v.a.r.i.a.*
+  00000020: 62 00 6C 00 65 00 00 00-                         *b.l.e...*
+Variable BS 'BB2A829F-7943-4691-A03A-F1F48519D7E6:MyVar' DataSize = 0x20
+  00000000: 4D 00 65 00 6D 00 6F 00-72 00 79 00 20 00 76 00  *M.e.m.o.r.y. .v.*
+  00000010: 61 00 72 00 69 00 61 00-62 00 6C 00 65 00 00 00  *a.r.i.a.b.l.e...*
+```
+Restart QEMU and issue `dmpstore` command:
+```
+FS0:\> dmpstore -guid bb2a829f-7943-4691-a03a-f1f48519d7e6
+Variable NV+BS 'BB2A829F-7943-4691-A03A-F1F48519D7E6:MyPersistentVar' DataSize = 0x28
+  00000000: 50 00 65 00 72 00 73 00-69 00 73 00 74 00 65 00  *P.e.r.s.i.s.t.e.*
+  00000010: 6E 00 74 00 20 00 76 00-61 00 72 00 69 00 61 00  *n.t. .v.a.r.i.a.*
+  00000020: 62 00 6C 00 65 00 00 00-                         *b.l.e...*
+```
+As you can see only the `MyPersistentVar` is present.
+
+Also your can check that is not possible to change attributes of an already created variable.
+```
+FS0:\> SetVariableExample.efi MyPersistentVar b "Persistent variable"
+Invalid Parameter
+```
+
+`dmpstore` command also can be used to delete variable
+```
+FS0:\> dmpstore -d -guid bb2a829f-7943-4691-a03a-f1f48519d7e6 MyPersistentVar
+Delete variable 'BB2A829F-7943-4691-A03A-F1F48519D7E6:MyPersistentVar': Success
+```
+
+You can delete all the variables behind the GUID with a `dmpstore -d <GUID>` command.
+
+In our application the variable content is a simple string. But keep in mind that this is just an example. Often variables have complicated structures stored in them.
+
+