On 02/02/18 15:07, Arnd Bergmann wrote:
In banked-sr.c, we use a top-level '__asm__(".arch_extension virt")'
statement to allow compilation of a multi-CPU kernel for ARMv6
and older ARMv7-A that don't normally support access to the banked
registers.

This is considered to be a programming error by the gcc developers
and will no longer work in gcc-8, where we now get a build error:

/tmp/cc4Qy7GR.s:34: Error: Banked registers are not available with this 
architecture. -- `mrs r3,SP_usr'
/tmp/cc4Qy7GR.s:41: Error: Banked registers are not available with this 
architecture. -- `mrs r3,ELR_hyp'
/tmp/cc4Qy7GR.s:55: Error: Banked registers are not available with this 
architecture. -- `mrs r3,SP_svc'
/tmp/cc4Qy7GR.s:62: Error: Banked registers are not available with this 
architecture. -- `mrs r3,LR_svc'
/tmp/cc4Qy7GR.s:69: Error: Banked registers are not available with this 
architecture. -- `mrs r3,SPSR_svc'
/tmp/cc4Qy7GR.s:76: Error: Banked registers are not available with this 
architecture. -- `mrs r3,SP_abt'

Passign the '-march-armv7ve' flag to gcc works, and is ok here, because
we know the functions won't ever be called on pre-ARMv7VE machines.
Unfortunately, older compiler versions (4.8 and earlier) do not understand
that flag, so we still need to keep the asm around.

Backporting to stable kernels (4.6+) is needed to allow those to be built
with future compilers as well.

Is "-Wa,arch=armv7-a+virt" (as we appear to do for a couple of files already) viable as a possibly cleaner alternative, or is GCC itself now policing the contents of inline asms?

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84129
Fixes: 33280b4cd1dc ("ARM: KVM: Add banked registers save/restore")
Cc: sta...@vger.kernel.org
Signed-off-by: Arnd Bergmann <a...@arndb.de>
---
  arch/arm/kvm/hyp/Makefile    | 5 +++++
  arch/arm/kvm/hyp/banked-sr.c | 4 ++++
  2 files changed, 9 insertions(+)

diff --git a/arch/arm/kvm/hyp/Makefile b/arch/arm/kvm/hyp/Makefile
index 5638ce0c9524..63d6b404d88e 100644
--- a/arch/arm/kvm/hyp/Makefile
+++ b/arch/arm/kvm/hyp/Makefile
@@ -7,6 +7,8 @@ ccflags-y += -fno-stack-protector -DDISABLE_BRANCH_PROFILING
KVM=../../../../virt/kvm +CFLAGS_ARMV7VE :=$(call cc-option, -march=armv7ve)
+
  obj-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/hyp/vgic-v2-sr.o
  obj-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/hyp/vgic-v3-sr.o
  obj-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/hyp/timer-sr.o
@@ -15,7 +17,10 @@ obj-$(CONFIG_KVM_ARM_HOST) += tlb.o
  obj-$(CONFIG_KVM_ARM_HOST) += cp15-sr.o
  obj-$(CONFIG_KVM_ARM_HOST) += vfp.o
  obj-$(CONFIG_KVM_ARM_HOST) += banked-sr.o
+CFLAGS_banked-sr.o        += $(CFLAGS_ARMV7VE)
+
  obj-$(CONFIG_KVM_ARM_HOST) += entry.o
  obj-$(CONFIG_KVM_ARM_HOST) += hyp-entry.o
  obj-$(CONFIG_KVM_ARM_HOST) += switch.o
+CFLAGS_switch.o                   += $(CFLAGS_ARMV7VE)
  obj-$(CONFIG_KVM_ARM_HOST) += s2-setup.o
diff --git a/arch/arm/kvm/hyp/banked-sr.c b/arch/arm/kvm/hyp/banked-sr.c
index 111bda8cdebd..be4b8b0a40ad 100644
--- a/arch/arm/kvm/hyp/banked-sr.c
+++ b/arch/arm/kvm/hyp/banked-sr.c
@@ -20,6 +20,10 @@
#include <asm/kvm_hyp.h> +/*
+ * gcc before 4.9 doesn't understand -march=armv7ve, so we have to
+ * trick the assembler.
+ */
  __asm__(".arch_extension     virt");

Would it be worth wrapping this in a preprocessor check for compilers that won't understand the command-line flag? I believe LLVM tends to choke on these global asm statements entirely, so minimising exposure might be a good thing to do in general.

Robin.

void __hyp_text __banked_save_state(struct kvm_cpu_context *ctxt)

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