Index: lib/Target/AMDGPU/AMDGPU.td
===================================================================
--- lib/Target/AMDGPU/AMDGPU.td
+++ lib/Target/AMDGPU/AMDGPU.td
@@ -73,6 +73,20 @@
   "Support unaligned scratch loads and stores"
 >;
 
+// XNACK is disabled if SH_MEM_CONFIG.ADDRESS_MODE = GPUVM. The current
+// default kernel driver setting is:
+// - graphics ring: XNACK disabled
+// - compute ring: XNACK enabled
+//                 (even on dGPUs - it's a kernel bug; just don't use the
+//                  compute ring or fix the kernel by making XNACK configurable
+//                  per IB or disable it completely)
+// - HSA queues: XNACK enabled (same issue as the compute ring)
+//
+// If XNACK is enabled, the VMEM latency can be worse, and that's an incentive
+// not to use the compute ring on targets that don't have to use it, like Mesa.
+//
+// If XNACK is disabled, the 2 SGPRs can be used for general purposes, which
+// mainly benefits Tonga and Iceland with their lower SGPR limit.
 def FeatureXNACK : SubtargetFeature<"xnack",
   "EnableXNACK",
   "true",
Index: lib/Target/AMDGPU/SIRegisterInfo.cpp
===================================================================
--- lib/Target/AMDGPU/SIRegisterInfo.cpp
+++ lib/Target/AMDGPU/SIRegisterInfo.cpp
@@ -1120,7 +1120,7 @@
       ST.isAmdHsaOS())
     return 6; // FLAT_SCRATCH, XNACK, VCC (in that order)
 
-  if (ST.getGeneration() >= AMDGPUSubtarget::VOLCANIC_ISLANDS)
+  if (ST.isXNACKEnabled())
     return 4; // XNACK, VCC (in that order)
 
   return 2; // VCC.
Index: test/CodeGen/AMDGPU/flat-scratch-reg.ll
===================================================================
--- test/CodeGen/AMDGPU/flat-scratch-reg.ll
+++ test/CodeGen/AMDGPU/flat-scratch-reg.ll
@@ -1,7 +1,10 @@
 ; RUN: llc -march=amdgcn -mcpu=kaveri -verify-machineinstrs < %s | FileCheck -check-prefix=NOXNACK -check-prefix=CI -check-prefix=GCN %s
 ; RUN: llc -march=amdgcn -mcpu=fiji -verify-machineinstrs < %s | FileCheck -check-prefix=NOXNACK -check-prefix=VI -check-prefix=GCN %s
-; RUN: llc -march=amdgcn -mcpu=carrizo -verify-machineinstrs < %s | FileCheck -check-prefix=XNACK -check-prefix=VI-XNACK  -check-prefix=GCN %s
-; RUN: llc -march=amdgcn -mcpu=stoney -verify-machineinstrs < %s | FileCheck -check-prefix=XNACK -check-prefix=VI-XNACK  -check-prefix=GCN %s
+; RUN: llc -march=amdgcn -mcpu=carrizo -mattr=-xnack -verify-machineinstrs < %s | FileCheck -check-prefix=NOXNACK -check-prefix=VI -check-prefix=GCN %s
+; RUN: llc -march=amdgcn -mcpu=stoney -mattr=-xnack -verify-machineinstrs < %s | FileCheck -check-prefix=NOXNACK -check-prefix=VI -check-prefix=GCN %s
+
+; RUN: llc -march=amdgcn -mcpu=carrizo -verify-machineinstrs < %s | FileCheck -check-prefix=XNACK -check-prefix=VI-XNACK -check-prefix=GCN %s
+; RUN: llc -march=amdgcn -mcpu=stoney -verify-machineinstrs < %s | FileCheck -check-prefix=XNACK -check-prefix=VI-XNACK -check-prefix=GCN %s
 
 ; RUN: llc -march=amdgcn -mtriple=amdgcn--amdhsa -mcpu=kaveri -verify-machineinstrs < %s | FileCheck -check-prefix=HSA-CI -check-prefix=NOXNACK -check-prefix=HSA-NOXNACK -check-prefix=HSA -check-prefix=GCN %s
 ; RUN: llc -march=amdgcn -mtriple=amdgcn--amdhsa -mcpu=carrizo -mattr=-xnack -verify-machineinstrs < %s | FileCheck -check-prefix=HSA-VI -check-prefix=NOXNACK -check-prefix=HSA-NOXNACK -check-prefix=HSA -check-prefix=GCN %s