Target the whole Cortex-X4 on the Steam Frame, which exposes SVE

The Frame's kernel reports sve, sve2 and the SVE2 extensions in its
HWCAP, on SteamOS and inside Lepton alike, unlike phones with the same
Snapdragon 8 Gen 3. The steamFrame flavour now builds with
-mcpu=cortex-x4 instead of cortex-x4+nosve.

docs/steam-frame.md records what the headset reported: 4 KB pages,
Android 11 (API 30) in a Waydroid-based Lepton, Mesa's Turnip as its
Vulkan driver, and SteamVR's native linuxarm64 OpenXR runtime on the
host, with the questions still open.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

android/Build-Quest.ps1

@@ -86,7 +86,7 @@ $variant = (Get-Culture).TextInfo.ToTitleCase($Configuration)
 # The Gradle flavour and the -mcpu target its kit must record (headsetCpus in app/build.gradle.kts).
 $flavourDir, $expectedCpu = switch ($Headset) {
     'quest1' { 'quest1', 'kryo' }
-    'frame' { 'steamFrame', 'cortex-x4+nosve' }
+    'frame' { 'steamFrame', 'cortex-x4' }
     default { 'modernQuest', 'cortex-a77' }
 }
 $flavour = $flavourDir.Substring(0, 1).ToUpperInvariant() + $flavourDir.Substring(1)

android/app/build.gradle.kts

@@ -61,9 +61,9 @@ val headsetCpus = mapOf(
     "modernQuest" to "cortex-a77",
     // Snapdragon 835. cortex-a77 binaries terminate with SIGILL on Quest 1.
     "quest1" to "kryo",
-    // Snapdragon 8 Gen 3 (Cortex-X4, A720 and A520, all ARMv9.2). Its firmware does not expose SVE,
-    // which clang would otherwise auto-vectorise with for a cortex-x4.
-    "steamFrame" to "cortex-x4+nosve",
+    // Steam Frame: Snapdragon 8 Gen 3 (Cortex-X4, A720 and A520, all ARMv9.2). Its kernel exposes
+    // SVE and SVE2 (HWCAP, inside Lepton too), so the whole cortex-x4 feature set is safe.
+    "steamFrame" to "cortex-x4",
 )
 
 // android/nod-jni: nod, the disc image library the PC installer runs as nodtool,

docs/steam-frame.md

@@ -18,7 +18,7 @@ device checks at the end are still to do.
 
 | | Quest flavours | `steamFrame` |
 | --- | --- | --- |
-| CPU target (`kit.json` `androidCpu`) | `cortex-a77` (`kryo` on Quest 1) | `cortex-x4+nosve` |
+| CPU target (`kit.json` `androidCpu`) | `cortex-a77` (`kryo` on Quest 1) | `cortex-x4` |
 | `MKW_ANDROID_HEADSET` | `quest` | `steam_frame` (defines `MKW_HEADSET_STEAM_FRAME`) |
 | Library entry | `LauncherActivity` (Quest 1: `QuestActivity`) | `FrameEntryActivity` |
 | Horizon OS manifest entries | present | removed |
@@ -31,11 +31,12 @@ as they are. The kit's CPU string differs from the Quest ones, which gives the F
 fingerprint: a game built for a Quest is refused on the Frame and the other way round, by the same
 checks that keep Quest 1 and modern Quest games apart.
 
-**CPU.** Every core of the 8 Gen 3 implements ARMv9.2, so the products are tuned for the Cortex-X4.
-`+nosve` matters: clang auto-vectorises with SVE for a `cortex-x4` (a simple loop compiled with
-`-O3` used SVE registers ten times), and Qualcomm's firmware does not expose SVE on this chip, so
-those instructions would end the game with `SIGILL`. With `+nosve` the target features read
-`-sve -sve2 -sve2-bitperm` and the same loop uses NEON only. The flavour-to-CPU map lives once in
+**CPU.** Every core of the 8 Gen 3 implements ARMv9.2, so the products target the Cortex-X4 with
+its whole feature set. That includes SVE and SVE2, which clang auto-vectorises with (a simple loop
+compiled with `-O3` used SVE registers ten times). Phones with this chip do not expose SVE, but the
+Frame's kernel does: `/proc/cpuinfo` lists `sve`, `sve2`, `svei8mm`, `svebf16` and the SVE2 crypto
+extensions, on SteamOS and inside Lepton alike. A build for a device without SVE would need
+`cortex-x4+nosve`. The flavour-to-CPU map lives once in
 `android/app/build.gradle.kts` (`headsetCpus`), which the kit export also reads now instead of
 guessing from the variant name.
 
@@ -131,7 +132,7 @@ the device's extension list.
 On the Windows build host described in `docs/quest-port.md`:
 
 ```powershell
-powershell -ExecutionPolicy Bypass -File android/Build-Quest.ps1 -Headset frame               # the steamFrame APK; checks kit.json says cortex-x4+nosve
+powershell -ExecutionPolicy Bypass -File android/Build-Quest.ps1 -Headset frame               # the steamFrame APK; checks kit.json says cortex-x4
 powershell -ExecutionPolicy Bypass -File android/Build-QuestGame.ps1 -Headset frame -Product base -Data <DATA>   # a .wcgame for the Frame's kit
 ```
 
@@ -145,6 +146,33 @@ network. With an Android app running on the Frame, `adb connect <frame-ip>:5555`
 build PC. How the APK reaches the Steam library (adb into a Lepton instance, frame-control, or
 Steam's own sideloading) is to be confirmed on the device.
 
+## What the Frame reported
+
+Read on 2026-10-04 from a Steam Frame running SteamOS (`holo`), kernel 6.18.0, with the commands
+below:
+
+| Reading | SteamOS | Lepton |
+| --- | --- | --- |
+| Page size | 4096 | 4096 |
+| CPU | 8 cores; `sve sve2 svei8mm svebf16 sveaes svepmull svebitperm svesha3 svesm4 i8mm bf16 bti paca pacg ...` | the same |
+| Android | — | 11 (API 30), `ro.product.model` Lepton, device `lepton_arm64_only`, platform `waydroid` |
+| Vulkan driver | — | `ro.hardware.vulkan=freedreno`: Mesa's Turnip, not Qualcomm's driver |
+| OpenXR runtime | SteamVR, `bin/linuxarm64/vrclient.so` (`~/.config/openxr/1/active_runtime.json`) | no package named for XR, Valve, Steam or Khronos |
+
+What follows from them:
+
+- **Fast memory path.** 4 KB pages keep the translated code's flat memory path. A 16 KB kernel
+  would have sent it through the checked path.
+- **CPU target.** The Frame exposes SVE, so the build targets the whole `cortex-x4` (above).
+- **Android version.** API 30 meets the app's minimum of 29.
+- **Driver workarounds.** Lepton is a Waydroid container, and its Vulkan driver is Turnip. The
+  Adreno workarounds in `docs/quest-port.md` were found on Qualcomm's own driver. The vertex padding
+  stays on (it is correct either way); `debug.wiicompiled.vtxpad 0` can check whether Turnip needs it.
+- **Open:** whether Turnip in Lepton imports AHardwareBuffers and sync fds is in the `vkjson`
+  extension list, still to read.
+- **Open:** how an app inside Lepton reaches SteamVR's OpenXR runtime. No broker package shows up,
+  so the Khronos loader's runtime broker may be provided some other way.
+
 ## Device checklist
 
 Information to collect first, from any PC over Lepton's adb port (the commands work in bash, zsh and