SingleRegisterAccess: Support unavailable regs

Enable an implementation of SingleRegisterAccess to signal to gdbstub
that a read register is recognized (as per RegId::from_raw_id()) but
is unavailable by reporting a 0-byte read.

In that case, make gdbstub report to the GDB client the state of the
unavailable register by following the registers packet protocol:

> [...] the stub may also return a string of literal `x`s in place of
> the register data digits, to indicate that the corresponding register
> has not been collected, thus its value is unavailable.

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>

examples/armv4t/gdb/mod.rs

@@ -314,6 +314,7 @@ impl target::ext::base::single_register_access::SingleRegisterAccess<()> for Emu
                 );
                 Ok(buf.len())
             }
+            custom_arch::ArmCoreRegIdCustom::Unavailable => Ok(0),
         }
     }
 
@@ -341,7 +342,8 @@ impl target::ext::base::single_register_access::SingleRegisterAccess<()> for Emu
                 Ok(())
             }
             // ignore writes
-            custom_arch::ArmCoreRegIdCustom::Time => Ok(()),
+            custom_arch::ArmCoreRegIdCustom::Unavailable
+            | custom_arch::ArmCoreRegIdCustom::Time => Ok(()),
         }
     }
 }
@@ -460,13 +462,16 @@ mod custom_arch {
         // not sent as part of `struct ArmCoreRegsCustom`, and only accessible via the single
         // register read/write functions
         Time,
+        /// This pseudo-register is valid but never available
+        Unavailable,
     }
 
     impl RegId for ArmCoreRegIdCustom {
         fn from_raw_id(id: usize) -> Option<(Self, Option<NonZeroUsize>)> {
             let reg = match id {
                 26 => Self::Custom,
                 27 => Self::Time,
+                28 => Self::Unavailable,
                 _ => {
                     let (reg, size) = ArmCoreRegId::from_raw_id(id)?;
                     return Some((Self::Core(reg), size));
@@ -530,13 +535,15 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Fps) => "padding",
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr) => "cpsr",
                         ArmCoreRegIdCustom::Custom => "custom",
+                        ArmCoreRegIdCustom::Unavailable => "Unavailable",
                         _ => "unknown",
                     };
                     let encoding = match r {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Gpr(_i)) => Encoding::Uint,
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Sp)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Pc)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr)
+                        | ArmCoreRegIdCustom::Unavailable
                         | ArmCoreRegIdCustom::Custom => Encoding::Uint,
                         _ => Encoding::Vector,
                     };
@@ -545,6 +552,7 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Sp)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Pc)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr)
+                        | ArmCoreRegIdCustom::Unavailable
                         | ArmCoreRegIdCustom::Custom => Format::Hex,
                         _ => Format::VectorUInt8,
                     };
@@ -555,6 +563,7 @@ mod custom_arch {
                         ArmCoreRegIdCustom::Core(ArmCoreRegId::Sp)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Pc)
                         | ArmCoreRegIdCustom::Core(ArmCoreRegId::Cpsr)
+                        | ArmCoreRegIdCustom::Unavailable
                         | ArmCoreRegIdCustom::Custom => "General Purpose Registers",
                         _ => "Floating Point Registers",
                     };

examples/armv4t/gdb/target_description_xml_override.rs

@@ -96,5 +96,13 @@ const EXTRA_XML: &str = r#"
         this register via the 'p' and 'P' packets respectively
     -->
     <reg name="time" bitsize="32" type="uint32"/>
+
+    <!--
+        pseudo-register that is always unavailable.
+
+        it is supposed to be reported as 'x'-ed bytes in replies to 'p' packets
+        and shown by the GDB client as "<unavailable>".
+    -->
+    <reg name="unavailable" bitsize="32" type="uint32"/>
 </feature>
 "#;

src/stub/core_impl/single_register_access.rs

@@ -33,14 +33,24 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
 
                 let len = ops.read_register(id, reg_id, buf).handle_error()?;
 
-                if let Some(size) = reg_size {
-                    if size.get() != len {
+                if len == 0 {
+                    if let Some(size) = reg_size {
+                        for _ in 0..size.get() {
+                            res.write_str("xx")?;
+                        }
+                    } else {
                         return Err(Error::TargetMismatch);
                     }
                 } else {
-                    buf = buf.get_mut(..len).ok_or(Error::PacketBufferOverflow)?;
+                    if let Some(size) = reg_size {
+                        if size.get() != len {
+                            return Err(Error::TargetMismatch);
+                        }
+                    } else {
+                        buf = buf.get_mut(..len).ok_or(Error::PacketBufferOverflow)?;
+                    }
+                    res.write_hex_buf(buf)?;
                 }
-                res.write_hex_buf(buf)?;
                 HandlerStatus::Handled
             }
             SingleRegisterAccess::P(p) => {

src/target/ext/base/single_register_access.rs

@@ -29,7 +29,8 @@ where
     /// Implementations should write the value of the register using target's
     /// native byte order in the buffer `buf`.
     ///
-    /// Return the number of bytes written into `buf`.
+    /// Return the number of bytes written into `buf` or `0` if the register is
+    /// valid but unavailable.
     ///
     /// If the requested register could not be accessed, an appropriate
     /// non-fatal error should be returned.