On 14/01/2016 09:30, Pavel Fedin wrote:
> Hello!
>
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -3359,6 +3359,14 @@ Hyper-V SynIC state change. Notification is used to remap SynIC
>> event/message pages and to enable/disable SynIC messages/events processing
>> in userspace.
>>
>> + /* KVM_EXIT_HYPERV_HCALL */
>> + struct {
>> + __u64 input;
>> + __u64 params[2];
>> + __u64 result;
>> + } hv_hcall;
>> +Indicates that the VCPU exits into userspace to process some guest
>> +Hyper-V hypercalls.
>
> Why introducing this? We already have KVM_EXIT_HYPERCALL, which is
> exactly the same. AFAIK it's not used at the moment.
> Additionally, in theory we could have hypercalls handled in
> userspace for something else except HyperV. And not only for x86.
Because, as the docs say, we don't want to do that. We want to use
KVM_EXIT_IO or KVM_EXIT_MMIO, with two exceptions: s390 and wherever we
can't do that for compatibility purposes. This is the latter. So we
should not add a new exit (I suggested elsewhere the existing hyper-v
exit) but we shouldn't use KVM_EXIT_HYPERCALL either.
There is a precedent in fact of using "something else than
KVM_EXIT_HYPERCALL" for interoperability/compatibility hypercalls,
namely KVM_EXIT_PAPR_HCALL.
Paolo
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