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Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy

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On 15/09/2015 18:13, Cornelia Huck wrote:
> On Tue, 15 Sep 2015 17:07:55 +0200
> Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
>> On 15/09/2015 08:41, Jason Wang wrote:
>>> +With KVM_CAP_FAST_MMIO, a zero length mmio eventfd is allowed for
>>> +kernel to ignore the length of guest write and get a possible faster
>>> +response. Note the speedup may only work on some specific
>>> +architectures and setups. Otherwise, it's as fast as wildcard mmio
>>> +eventfd.
>>
>> I don't really like tying the capability to MMIO, especially since
>> zero length ioeventfd is already accepted for virtio-ccw.
> 
> Actually, zero length ioeventfd does not make sense for virtio-ccw;

Can you explain why?  If there is any non-zero valid length, "wildcard
length" (represented by zero) would also make sense.

Paolo

> we just don't check it (although we probably should).
> 
>>
>> What about the following?
>>
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> index 7a3cb48a644d..247944071cc8 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -1627,11 +1627,10 @@ to the registered address is equal to datamatch in struct kvm_ioeventfd.
>>  For virtio-ccw devices, addr contains the subchannel id and datamatch the
>>  virtqueue index.
>>
>> -With KVM_CAP_FAST_MMIO, a zero length mmio eventfd is allowed for
>> -kernel to ignore the length of guest write and get a possible faster
>> -response. Note the speedup may only work on some specific
>> -architectures and setups. Otherwise, it's as fast as wildcard mmio
>> -eventfd.
>> +With KVM_CAP_IOEVENTFD_ANY_LENGTH, a zero length ioeventfd is allowed, and
>> +the kernel will ignore the length of guest write and get a faster vmexit.
> 
> s/get/may get/ ?
> 
>> +The speedup may only apply to specific architectures, but the ioeventfd will
>> +work anyway.
> 
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[PATCH V6 0/6] Fast mmio eventfd fixes Jason Wang <jasowang@redhat.com> - 2015-09-15 08:50 +0200
  [PATCH V6 6/6] kvm: add fast mmio capabilitiy Jason Wang <jasowang@redhat.com> - 2015-09-15 08:50 +0200
    Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy Paolo Bonzini <pbonzini@redhat.com> - 2015-09-15 17:10 +0200
      Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy Cornelia Huck <cornelia.huck@de.ibm.com> - 2015-09-15 18:20 +0200
        Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy Paolo Bonzini <pbonzini@redhat.com> - 2015-09-15 18:30 +0200
          Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy Cornelia Huck <cornelia.huck@de.ibm.com> - 2015-09-15 18:50 +0200
            Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy Paolo Bonzini <pbonzini@redhat.com> - 2015-09-15 18:50 +0200
  [PATCH V6 2/6] kvm: factor out core eventfd assign/deassign logic Jason Wang <jasowang@redhat.com> - 2015-09-15 08:50 +0200
    Re: [PATCH V6 2/6] kvm: factor out core eventfd assign/deassign  logic Cornelia Huck <cornelia.huck@de.ibm.com> - 2015-09-15 09:10 +0200
  [PATCH V6 3/6] kvm: fix double free for fast mmio eventfd Jason Wang <jasowang@redhat.com> - 2015-09-15 08:50 +0200
    Re: [PATCH V6 3/6] kvm: fix double free for fast mmio eventfd Cornelia Huck <cornelia.huck@de.ibm.com> - 2015-09-15 09:20 +0200
  Re: [PATCH V6 0/6] Fast mmio eventfd fixes Paolo Bonzini <pbonzini@redhat.com> - 2015-09-15 17:10 +0200
    Re: [PATCH V6 0/6] Fast mmio eventfd fixes "Michael S. Tsirkin" <mst@redhat.com> - 2015-09-15 21:30 +0200
      Re: [PATCH V6 0/6] Fast mmio eventfd fixes Paolo Bonzini <pbonzini@redhat.com> - 2015-09-16 11:10 +0200

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