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

["Wang,Guangju" <wangguangju@baidu.com>]

>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.

static void
__x2apic_send_IPI_mask(const struct cpumask *mask, int vector, int apic_dest)
{
	unsigned long query_cpu;
	unsigned long this_cpu;
	unsigned long flags;

	/* x2apic MSRs are special and need a special fence: */
	weak_wrmsr_fence();

	local_irq_save(flags);

	this_cpu = smp_processor_id();
	for_each_cpu(query_cpu, mask) {
		if (apic_dest == APIC_DEST_ALLBUT && this_cpu == query_cpu)
			continue;
		__x2apic_send_IPI_dest(per_cpu(x86_cpu_to_apicid, query_cpu),
				       vector, APIC_DEST_PHYSICAL);
	}
	local_irq_restore(flags);
}