Re: [PATCH v3 1/2] KVM: x86: Check hypercall's exit to userspace generically

[Binbin Wu <binbin.wu@linux.intel.com>]

On 11/4/2024 5:55 PM, Huang, Kai wrote:
[...]
>>>    int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>>>    {
>>> -	unsigned long nr, a0, a1, a2, a3, ret;
>>> -	int op_64_bit;
>>> -	int cpl;
>>> +	int r;
>>>    
>>>    	if (kvm_xen_hypercall_enabled(vcpu->kvm))
>>>    		return kvm_xen_hypercall(vcpu);
>>> @@ -10102,23 +10105,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>>>    	if (kvm_hv_hypercall_enabled(vcpu))
>>>    		return kvm_hv_hypercall(vcpu);
>>>    
>>> -	nr = kvm_rax_read(vcpu);
>>> -	a0 = kvm_rbx_read(vcpu);
>>> -	a1 = kvm_rcx_read(vcpu);
>>> -	a2 = kvm_rdx_read(vcpu);
>>> -	a3 = kvm_rsi_read(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. */
>>> +	r = __kvm_emulate_hypercall(vcpu, rax, rbx, rcx, rdx, rsi,
>>> +				    is_64_bit_hypercall(vcpu),
>>> +				    kvm_x86_call(get_cpl)(vcpu), RAX);
>> Now, the register for return code of the hypercall can be specified.
>> But in  ____kvm_emulate_hypercall(), the complete_userspace_io callback
>> is hardcoded to complete_hypercall_exit(), which always set return code
>> to RAX.
>>
>> We can allow the caller to pass in the cui callback, or assign different
>> version according to the input 'ret_reg'.  So that different callers can use
>> different cui callbacks.  E.g., TDX needs to set return code to R10 in cui
>> callback.
>>
>> How about:
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index dba78f22ab27..0fba98685f42 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -2226,13 +2226,15 @@ static inline void kvm_clear_apicv_inhibit(struct kvm *kvm,
>>    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 ret_reg);
>> +                             int op_64_bit, int cpl, int ret_reg,
>> +                             int (*cui)(struct kvm_vcpu *vcpu));
>>
> Does below (incremental diff based on Sean's) work?

Yes, it can work.

>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 734dac079453..5131af97968d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10075,7 +10075,6 @@ int ____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;
>                  /* stat is incremented on completion. */
>                  return 0;
>          }
> @@ -10108,8 +10107,11 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>          r = __kvm_emulate_hypercall(vcpu, rax, rbx, rcx, rdx, rsi,
>                                      is_64_bit_hypercall(vcpu),
>                                      kvm_x86_call(get_cpl)(vcpu), RAX);
> -       if (r <= 0)
> +       if (r <= 0) {
> +               if (!r)
> +                       vcpu->arch.complete_userspace_io =
> complete_hypercall_exit;
>                  return 0;
> +       }
>   
>          return kvm_skip_emulated_instruction(vcpu);
>   }
>