i686: add layout, vmem, etc stubs

As per the comments preexisting in
src/hyperlight_common/src/layout.rs. This avoids some fairly
cumbersome proliferation of #[cfg(feature = "init-paging")], and may
be developed further in the future.

Signed-off-by: Lucy Menon <168595099+syntactically@users.noreply.github.com>

Author: syntactically

src/hyperlight_common/src/arch/i686/layout.rs

@@ -0,0 +1,22 @@
+/*
+Copyright 2025  The Hyperlight Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+ */
+
+// This file is just dummy definitions at the moment, in order to
+// allow compiling the guest for real mode boot scenarios.
+
+pub const MAX_GVA: usize = 0xffff_efff;
+pub const SNAPSHOT_PT_GVA: usize = 0xefff_efff;
+pub const MAX_GPA: usize = 0xffff_ffff;

src/hyperlight_common/src/arch/i686/vmem.rs

@@ -0,0 +1,38 @@
+/*
+Copyright 2025  The Hyperlight Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+ */
+
+// This file is just dummy definitions at the moment, in order to
+// allow compiling the guest for real mode boot scenarios.
+
+use crate::vmem::{BasicMapping, Mapping, TableOps};
+
+pub const PAGE_SIZE: usize = 4096;
+pub const PAGE_TABLE_SIZE: usize = 4096;
+pub type PageTableEntry = u32;
+pub type VirtAddr = u32;
+pub type PhysAddr = u32;
+
+#[allow(clippy::missing_safety_doc)]
+pub unsafe fn map<Op: TableOps>(_op: &Op, _mapping: Mapping) {
+    panic!("vmem::map: i686 guests do not support booting the full hyperlight guest kernel");
+}
+
+#[allow(clippy::missing_safety_doc)]
+pub unsafe fn virt_to_phys<Op: TableOps>(_op: &Op, _address: u64) -> Option<(u64, BasicMapping)> {
+    panic!(
+        "vmem::virt_to_phys: i686 guests do not support booting the full hyperlight guest kernel"
+    );
+}

src/hyperlight_common/src/layout.rs

@@ -14,28 +14,21 @@ See the License for the specific language governing permissions and
 limitations under the License.
  */
 
-// The constraint on the feature is temporary and will be removed when other arch i686 is added
 #[cfg_attr(target_arch = "x86_64", path = "arch/amd64/layout.rs")]
-#[cfg(feature = "init-paging")]
+#[cfg_attr(target_arch = "x86", path = "arch/i686/layout.rs")]
 mod arch;
 
-// The constraint on the feature is temporary and will be removed when other arch i686 is added
-#[cfg(feature = "init-paging")]
-pub use arch::MAX_GPA;
-#[cfg(feature = "init-paging")]
-pub use arch::{MAX_GVA, SNAPSHOT_PT_GVA};
+pub use arch::{MAX_GPA, MAX_GVA, SNAPSHOT_PT_GVA};
 
 // offsets down from the top of scratch memory for various things
 pub const SCRATCH_TOP_SIZE_OFFSET: u64 = 0x08;
 pub const SCRATCH_TOP_USED_OFFSET: u64 = 0x10;
 pub const SCRATCH_TOP_ALLOCATOR_OFFSET: u64 = 0x18;
 pub const SCRATCH_TOP_EXN_STACK_OFFSET: u64 = 0x20;
 
-#[cfg(feature = "init-paging")]
 pub fn scratch_base_gpa(size: usize) -> u64 {
     (MAX_GPA - size + 1) as u64
 }
-#[cfg(feature = "init-paging")]
 pub fn scratch_base_gva(size: usize) -> u64 {
     (MAX_GVA - size + 1) as u64
 }

src/hyperlight_common/src/lib.rs

@@ -41,6 +41,6 @@ pub mod resource;
 
 /// cbindgen:ignore
 pub mod func;
+
 // cbindgen:ignore
-#[cfg(feature = "init-paging")]
 pub mod vmem;

src/hyperlight_common/src/vmem.rs

@@ -15,9 +15,13 @@ limitations under the License.
  */
 
 #[cfg_attr(target_arch = "x86_64", path = "arch/amd64/vmem.rs")]
+#[cfg_attr(target_arch = "x86", path = "arch/i686/vmem.rs")]
 pub mod arch;
 
-pub use arch::{PAGE_SIZE, PAGE_TABLE_SIZE, PageTableEntry, PhysAddr, VirtAddr};
+/// This is always the page size that the /guest/ is being compiled
+/// for, which may or may not be the same as the host page size.
+pub use arch::PAGE_SIZE;
+pub use arch::{PAGE_TABLE_SIZE, PageTableEntry, PhysAddr, VirtAddr};
 pub const PAGE_TABLE_ENTRIES_PER_TABLE: usize =
     PAGE_TABLE_SIZE / core::mem::size_of::<PageTableEntry>();
 

src/hyperlight_guest/src/arch/i686/layout.rs

@@ -0,0 +1,32 @@
+/*
+Copyright 2025  The Hyperlight Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+ */
+
+// This file is just dummy definitions at the moment, in order to
+// allow compiling the guest for real mode boot scenarios.
+
+pub const MAIN_STACK_TOP_GVA: usize = 0xdfff_efff;
+
+pub fn scratch_size() -> u64 {
+    hyperlight_common::vmem::PAGE_SIZE as u64
+}
+
+pub fn scratch_base_gpa() -> u64 {
+    hyperlight_common::layout::scratch_base_gpa(scratch_size() as usize)
+}
+
+pub fn scratch_base_gva() -> u64 {
+    hyperlight_common::layout::scratch_base_gva(scratch_size() as usize)
+}

src/hyperlight_guest/src/arch/i686/prim_alloc.rs

@@ -0,0 +1,25 @@
+/*
+Copyright 2025  The Hyperlight Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+ */
+
+// This file is just dummy definitions at the moment, in order to
+// allow compiling the guest for real mode boot scenarios.
+
+#[allow(clippy::missing_safety_doc)]
+pub unsafe fn alloc_phys_pages(_n: u64) -> u64 {
+    panic!(
+        "prim_alloc::alloc_phys_pages: i686 guests do not support booting the full hyperlight guest kernel"
+    );
+}

src/hyperlight_guest/src/layout.rs

@@ -15,6 +15,7 @@ limitations under the License.
  */
 
 #[cfg_attr(target_arch = "x86_64", path = "arch/amd64/layout.rs")]
+#[cfg_attr(target_arch = "x86", path = "arch/i686/layout.rs")]
 mod arch;
 
 pub use arch::MAIN_STACK_TOP_GVA;

src/hyperlight_guest/src/prim_alloc.rs

@@ -15,6 +15,7 @@ limitations under the License.
  */
 
 #[cfg_attr(target_arch = "x86_64", path = "arch/amd64/prim_alloc.rs")]
+#[cfg_attr(target_arch = "x86", path = "arch/i686/prim_alloc.rs")]
 mod arch;
 
 /// Allocate n contiguous physical pages and return the physical