RFC: Drop MSVC AARCH64 Support

This commit adds the aforementioned RFC.

Signed-off-by: Oliver Smith-Denny <osde@microsoft.com>

Author: os-d

rfc/text/0000-drop-msvc-aarch64-support.md

@@ -0,0 +1,121 @@
+# RFC: Drop MSVC AARCH64 Support
+
+## Metadata
+
+- **RFC Number**: TBD
+- **Title**: Drop MSVC AARCH64 Support
+- **Status**: Draft
+
+## Change Log
+
+- 2026-07-13: Initial RFC created
+
+## Motivation
+
+edk2 has build bindings for MSVC AARCH64 but lacks support for this toolchain in most
+of the repository. The toolchain is not exercised in CI and a full platform cannot be built with
+it without significant downstream overrides. One large blocker is that GAS assembly is not supported
+by the toolchain, but the majority of the AARCH64 assembly code in edk2 is GAS only.
+
+The benefit of MSVC AARCH64 is that it natively produces PE/COFF binaries (and so thereby allowing
+different file and section alignment, PDB debugging, and less processing). However, the newly added
+CLANGPDB AARCH64 also natively produces PE/COFF binaries, is supported on Linux and Windows, uses
+GAS assembly, and is run in CI today.
+
+Pseudo supporting additional toolchains carries maintenance burden and risk, especially as this
+toolchain will never be tested. Consumers should move to CLANGPDB AARCH64 instead and MSVC
+AARCH64 should be removed.
+
+## Technology Background
+
+CLANGPDB AARCH64 support was added to edk2 in https://github.com/tianocore/edk2/pull/11902 in
+early 2026. Prior to this, MSVC AARCH64 was the only toolchain with edk2 bindings for AARCH64 that
+supported PE/COFF binary generation natively. However, all of the build support was primarily
+downstream and toolchain adoption never really grew, with GCC AARCH64 being the primary AARCH64
+toolchain in edk2.
+
+## Goals
+
+1. Reduce toolchain maintenance
+2. Remove dead/almost dead/forever untested code
+
+## Requirements
+
+1. Have a better alternative for consumers to migrate to
+
+## UEFI/PI Specification Impact
+
+This does not have specification impact.
+
+## Backward Compatibility
+
+This is a breaking change. edk2 will not support building with MSVC AARCH64 any longer. However,
+as noted, a full platform cannot be built with MSVC AARCH64 with edk2 anyway. Small usecases, such
+as an EFI_APPLICATION may be able to build.
+
+Consumers are expected to move to CLANGPDB AARCH64, which is a better alternative for the reasons
+listed above. GCC AARCH64 also exists as a possibility, but does not run on Windows, where MSVC
+AARCH64 developers would be.
+
+## Platform/Package Impact
+
+All.
+
+## Unresolved Questions
+
+N/A.
+
+## Prior Art/Related Work
+
+Toolchains have been deprecated in edk2 before, but primarily older versions rather than
+an entire toolchain.
+
+## Alternatives
+
+### Alternative 1: Do Nothing
+
+- **Pros**: No breaking change.
+- **Cons**: Unused code is kept in the tree, there is the appearance of support for what will likely
+            be a painful experience for consumers.
+- **Why not chosen**: Consumers will be better served by moving to the actually supported CLANGPDB AARCH64.
+                      edk2 maintenance will be improved by removing unused code.
+
+## Implementation Design
+
+The toolchain and supporting arm64asm files will be removed.
+
+### Architecture Overview
+
+N/A.
+
+### Detailed Design
+
+N/A.
+
+### Code Examples
+
+N/A.
+
+## Testing Strategy
+
+N/A.
+
+## Migration/Adoption Plan
+
+1. **Phase 1**: Announce deprecation of the toolchain
+2. **Phase 2**: Remove the toolchain
+
+## Guide-Level Explanation
+
+### For Package Developers
+
+MSVC AARCH64 is no longer supported. Migrate to CLANGPDB AARCH64 instead.
+
+### For Platform Developers
+
+MSVC AARCH64 is already unsupported for platform developers, but the
+same guidance applies.
+
+### For End Users (if applicable)
+
+N/A.