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

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

On Tue, Jun 14, 2022 at 02:41:30PM +0000, Sean Christopherson wrote:
>> 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.
>> 
>> The question actually is how to deal with the exceptional case below.
>> Considering the migration case Sean said below, it is hard to let VM
>> always work in the ideal way unless KVM notifies VM of migration and VM
>> changes its behavior on receiving such notifications. But since x2apic
>> has better performance than xapic, if VM cares about performance, it can
>> simply switch to x2apic mode. All things considered, I think the
>> performance gain isn't worth the complexity added. So, I prefer to leave
>> it as is.
>> 
>> [*]: when linux guest is in *xapic* mode, it uses PVIPI to send uni-case IPI.
>
>Hmm, there are definitely guests that run xAPIC though, even if x2apic is supported.
>
>That said, I tend to agree that trying to handle this in KVM and/or the guest kernel
>is going to get messy.  The easiest solution is for VMMs to not advertise PV IPIs
>when the VM is going to predominately run on hosts with IPIv.

But it will hurt multi-cast IPIs in VMs. IMO, a feasible solution is to add
a new hint to indicate IPI virtualization is enabled and VM uses native
interface (writes to ICR) to send uni-cast IPIs in xapic mode if it sees
the new hint.