Re: [PATCH v4 5/6] KVM: x86: Always complete hypercall via function callback

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 11/28/2024 8:43 AM, Sean Christopherson wrote:
> Finish "emulation" of KVM hypercalls by function callback, even when the
> hypercall is handled entirely within KVM, i.e. doesn't require an exit to
> userspace, and refactor __kvm_emulate_hypercall()'s return value to *only*
> communicate whether or not KVM should exit to userspace or resume the
> guest.
> 
> (Ab)Use vcpu->run->hypercall.ret to propagate the return value to the
> callback, purely to avoid having to add a trampoline for every completion
> callback.
> 
> Using the function return value for KVM's control flow eliminates the
> multiplexed return value, where '0' for KVM_HC_MAP_GPA_RANGE (and only
> that hypercall) means "exit to userspace".
> 
> Note, the unnecessary extra indirect call and thus potential retpoline
> will be eliminated in the near future by converting the intermediate layer
> to a macro.
> 
> Suggested-by: Binbin Wu <binbin.wu@linux.intel.com>
> Suggested-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/x86.c | 29 ++++++++++++-----------------
>   arch/x86/kvm/x86.h | 10 ++++++----
>   2 files changed, 18 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 11434752b467..39be2a891ab4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9982,10 +9982,11 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
>   	return kvm_skip_emulated_instruction(vcpu);
>   }
>   
> -unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> -				      unsigned long a0, unsigned long a1,
> -				      unsigned long a2, unsigned long a3,
> -				      int op_64_bit, int cpl)
> +int __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> +			    unsigned long a0, unsigned long a1,
> +			    unsigned long a2, unsigned long a3,
> +			    int op_64_bit, int cpl,
> +			    int (*complete_hypercall)(struct kvm_vcpu *))
>   {
>   	unsigned long ret;
>   
> @@ -10061,7 +10062,7 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>   			vcpu->run->hypercall.flags |= KVM_EXIT_HYPERCALL_LONG_MODE;
>   
>   		WARN_ON_ONCE(vcpu->run->hypercall.flags & KVM_EXIT_HYPERCALL_MBZ);
> -		vcpu->arch.complete_userspace_io = complete_hypercall_exit;
> +		vcpu->arch.complete_userspace_io = complete_hypercall;
>   		/* stat is incremented on completion. */
>   		return 0;
>   	}
> @@ -10071,13 +10072,15 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>   	}
>   
>   out:
> -	return ret;
> +	vcpu->run->hypercall.ret = ret;
> +	complete_hypercall(vcpu);
> +	return 1;

shouldn't it be

	return complete_hypercall(vcpu);

?

Originally, kvm_emulate_hypercall() returns 
kvm_skip_emulated_instruction(). Now it becomes

	kvm_skip_emulated_instruction();
	return 1;

I don't go deep to see if kvm_skip_emulated_instruction() always return 
1 for this case. But logically,

	return complete_hypercall(vcpu);

looks more reasonable.

>   }
>   EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);
>   
>   int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>   {
> -	unsigned long nr, a0, a1, a2, a3, ret;
> +	unsigned long nr, a0, a1, a2, a3;
>   	int op_64_bit;
>   	int cpl;
>   
> @@ -10095,16 +10098,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>   	op_64_bit = is_64_bit_hypercall(vcpu);
>   	cpl = kvm_x86_call(get_cpl)(vcpu);
>   
> -	ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
> -	if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
> -		/* MAP_GPA tosses the request to the user space. */
> -		return 0;
> -
> -	if (!op_64_bit)
> -		ret = (u32)ret;
> -	kvm_rax_write(vcpu, ret);
> -
> -	return kvm_skip_emulated_instruction(vcpu);
> +	return __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl,
> +				       complete_hypercall_exit);
>   }
>   EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
>   
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 6db13b696468..28adc8ea04bf 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -617,10 +617,12 @@ static inline bool user_exit_on_hypercall(struct kvm *kvm, unsigned long hc_nr)
>   	return kvm->arch.hypercall_exit_enabled & BIT(hc_nr);
>   }
>   
> -unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> -				      unsigned long a0, unsigned long a1,
> -				      unsigned long a2, unsigned long a3,
> -				      int op_64_bit, int cpl);
> +int __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> +			    unsigned long a0, unsigned long a1,
> +			    unsigned long a2, unsigned long a3,
> +			    int op_64_bit, int cpl,
> +			    int (*complete_hypercall)(struct kvm_vcpu *));
> +
>   int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
>   
>   #endif