Re: [PATCH 2/3] arm64: KVM: Allow for direct call of HYP functions when using VHE

[Marc Zyngier <marc.zyngier@arm.com>]

On 09/01/2019 14:51, Julien Thierry wrote:
> 
> 
> On 09/01/2019 14:45, Marc Zyngier wrote:
>> Hi Andrew,
>>
>> On 09/01/2019 14:24, Andrew Murray wrote:
>>> On Wed, Jan 09, 2019 at 01:54:34PM +0000, Marc Zyngier wrote:
>>>> When running VHE, there is no need to jump via some stub to perform
>>>> a "HYP" function call, as there is a single address space.
>>>>
>>>> Let's thus change kvm_call_hyp() and co to perform a direct call
>>>> in this case. Although this results in a bit of code expansion,
>>>> it allows the compiler to check for type compatibility, something
>>>> that we are missing so far.
>>>>
>>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>>>> ---
>>>>  arch/arm64/include/asm/kvm_host.h | 32 +++++++++++++++++++++++++++++--
>>>>  1 file changed, 30 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>>>> index e54cb7c88a4e..df32edbadd69 100644
>>>> --- a/arch/arm64/include/asm/kvm_host.h
>>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>>> @@ -370,8 +370,36 @@ void kvm_arm_halt_guest(struct kvm *kvm);
>>>>  void kvm_arm_resume_guest(struct kvm *kvm);
>>>>  
>>>>  u64 __kvm_call_hyp(void *hypfn, ...);
>>>> -#define kvm_call_hyp(f, ...) __kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__)
>>>> -#define kvm_call_hyp_ret(f, ...) kvm_call_hyp(f, ##__VA_ARGS__)
>>>> +
>>>> +/*
>>>> + * The couple of isb() below are there to guarantee the same behaviour
>>>> + * on VHE as on !VHE, where the eret to EL1 acts as a context
>>>> + * synchronization event.
>>>> + */
>>>> +#define kvm_call_hyp(f, ...)						\
>>>> +	do {								\
>>>> +		if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN)) {	\
>>>> +			f(__VA_ARGS__);					\
>>>> +			isb();						\
>>>> +		} else {						\
>>>> +			__kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__); \
>>>> +		}							\
>>>> +	} while(0)
>>>> +
>>>> +#define kvm_call_hyp_ret(f, ...)					\
>>>> +	({								\
>>>> +		u64 ret;						\
>>>> +									\
>>>> +		if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN)) {	\
>>>> +			ret = f(__VA_ARGS__);				\
>>>
>>> __kvm_get_mdcr_el2 and __kvm_vcpu_run_nvhe don't return u64 type, they
>>> return a smaller type. I guess any issues would be picked up when compiling,
>>> but should the name of the macro be clearer as to the assumptions it makes?
>>> Or perhaps take an argument which is the type of ret?
>>
>> kvm_call_hyp has always returned a u64, so no semantic has changed here.
>>
>> Otherwise, your suggestion of specifying a return type is interesting,
>> but it also gives the programmer another chance to shoot itself in the
>> foot by not providing the return type corresponding to the function that
>> is called.
>>
>> Unless we can extract the return type by pure magic, I'm not sure we
>> gain much.
>>
> 
> Would the following work?
> 
> 	typeof(f(__VA_ARGS__)) ret;
> 
> If typeof works anything like sizeof, I'd expect it would evaluate stuff
> passed as argument and we'd have the return type of the function.
And it actually works! Thanks for the awful tip! ;-)

	M.
-- 
Jazz is not dead. It just smells funny...