Re: 答复: [PATCH] KVM: x86: add a bool variable to distinguish whether to use PVIPI

[Chao Gao <chao.gao@intel.com>]

On Wed, Jun 15, 2022 at 04:21:21AM +0000, Wang,Guangju wrote:
>>On Mon, Jun 13, 2022 at 05:16:48PM +0000, Sean Christopherson wrote:
>>>The shortlog is not at all helpful, it doesn't say anything about what 
>>>actual functional change.
>>>
>>>  KVM: x86: Don't advertise PV IPI to userspace if IPIs are virtualized
>>>
>>>On Mon, Jun 13, 2022, wangguangju wrote:
>>>> Commit d588bb9be1da ("KVM: VMX: enable IPI virtualization") enable 
>>> >IPI virtualization in Intel SPR platform.There is no point in using 
>>> >PVIPI if IPIv is supported, it doesn't work less good with PVIPI than 
>>> >without it.
>>>> 
>>> >So add a bool variable to distinguish whether to use PVIPI.
>>>
>>>Similar complaint with the changelog, it doesn't actually call out why 
>>>PV IPIs are unwanted.
>>>
>>>  Don't advertise PV IPI support to userspace if IPI virtualization is  
>> >supported by the CPU.  Hardware virtualization of IPIs more performant  
>> >as senders do not need to exit.
>
>>PVIPI is mainly [*] for sending multi-cast IPIs. Intel IPI virtualization can virtualize only uni-cast IPIs. Their use cases don't overlap. So, I don't think it makes sense to disable PVIPI if intel IPI virtualization is supported.
>A question, like x2apic mode, guest uses PVIPI with replace apic->send_IPI_mask to kvm_send_ipi_mask. The original function implementation is __x2apic_send_IPI_mask , and it poll each CPU to send IPI. So in this case 
>Intel virtualization can not work? Thanks.

Yes, it can work. But some experiments we conducted based on a modified
kvm-unit-test showed that PVIPI outperforms native ICR writes (w/ IPI
virtualization) in terms of sending multi-cast (i.e., dest vCPUs >=2) IPIs