Fix the guest page size at compile time for the Steam Frame build

Every translated guest load and store tests RequiresCheckedAccess(). On
x86-64 and Windows that is a compile-time false, but on AArch64 it read
a global flag, which under PIE is a GOT load plus a byte load and a
branch on every access, re-read after every guest store. The flag is
only true on kernels with pages larger than 4 KiB.

The Steam Frame's SteamOS kernel uses 4 KiB pages (getconf PAGESIZE on
the headset reports 4096), so the native --headset steam_frame build now
folds the test away like x86-64 does. GuestFlat::Initialize checks the
page size at startup in that build and refuses to start with a clear
message if it is ever anything else. Android and other AArch64 builds
still probe at runtime.

Checked with clang --target=aarch64-linux-gnu -O2 -fPIE: a FlatRead32
loop loses both flag loads and the checked Memory::Read32 fallback.

Follows the finding from nx-mod/wiicompiled-nx 18ef16f6, which made the
flag hidden instead; a compile-time constant removes it entirely.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018NKSMYZU43wUfYtm1sxGsC

runtime/include/guest_flat_memory.h

@@ -71,10 +71,15 @@ bool IsActive();
 // accesses must use the checked Memory::* path.
 // Windows user mode and x86-64 always use a 4 KiB base page, so those builds
 // fold this to a compile-time false: it appears in every flat access and must
-// not become a hot-path load. Only AArch64, where the page size is a kernel
-// configuration (4/16/64 KiB), has to probe it at runtime.
+// not become a hot-path load. On AArch64 the page size is a kernel
+// configuration (4/16/64 KiB), so it is probed at runtime, except in the native
+// Steam Frame build: that build only runs on the headset, whose SteamOS kernel
+// uses 4 KiB pages, and Initialize() refuses to start on any other page size.
 #if defined(_WIN32) || defined(__x86_64__)
 #define MKW_GUEST_FLAT_FIXED_PAGE_SIZE 1
+#elif defined(MKW_HEADSET_STEAM_FRAME) && !defined(__ANDROID__)
+#define MKW_GUEST_FLAT_FIXED_PAGE_SIZE 1
+#define MKW_GUEST_FLAT_VERIFY_PAGE_SIZE 1
 #endif
 
 #if defined(MKW_GUEST_FLAT_FIXED_PAGE_SIZE)

runtime/src/guest_flat_memory.cpp

@@ -11,6 +11,7 @@
 #include <mutex>
 #include <sstream>
 #include <stdexcept>
+#include <string>
 #include <unordered_map>
 
 #include "memory.h"
@@ -61,8 +62,9 @@ constexpr size_t kAllocationGranularity = 0x10000;  // 64 KiB
 constexpr size_t kHostPageSize = 0x1000;
 
 // Only hosts that can expose a page larger than 4 KiB need to discover their
-// size at runtime; see RequiresCheckedAccess() in guest_flat_memory.h.
-#if !defined(MKW_GUEST_FLAT_FIXED_PAGE_SIZE)
+// size at runtime, or check it when the build assumes 4 KiB; see
+// RequiresCheckedAccess() in guest_flat_memory.h.
+#if !defined(MKW_GUEST_FLAT_FIXED_PAGE_SIZE) || defined(MKW_GUEST_FLAT_VERIFY_PAGE_SIZE)
 size_t HostPageSize()
 {
     const long size = sysconf(_SC_PAGESIZE);
@@ -521,6 +523,13 @@ void Initialize(const std::vector<RegionRequest>& regions) {
 
 #if !defined(MKW_GUEST_FLAT_FIXED_PAGE_SIZE)
     g_requiresCheckedAccess = HostPageSize() > kGuestPageSize;
+#elif defined(MKW_GUEST_FLAT_VERIFY_PAGE_SIZE)
+    if (HostPageSize() != kGuestPageSize) {
+        throw std::runtime_error(
+            "This is a Steam Frame build, which assumes the headset's 4 KiB memory pages, but this "
+            "kernel uses " + std::to_string(HostPageSize()) + "-byte pages. Build without "
+            "--headset steam_frame for this device.");
+    }
 #endif
 
     if (g_initialized) {