Re: [PATCH 1/2] ARM: kvm: fix building with gcc-8

[Robin Murphy <robin.murphy@arm.com>]

On 02/02/18 15:55, Robin Murphy wrote:
> 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?

In fact, looking at the binutils history, any version capable of 
assembling this file should understand that (modulo my typo), so 
hopefully it ought to be feasible to replace these global asms with 
assembler flags entirely.

Robin.

>> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84129
>> Fixes: 33280b4cd1dc ("ARM: KVM: Add banked registers save/restore")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Arnd Bergmann <arnd@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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