Re: [PATCH v4 3/3] x86/tlb: add architecture-specific TLB IPI optimization support

[Lance Yang <lance.yang@linux.dev>]

On 2026/2/26 04:11, Sean Christopherson wrote:
> On Mon, Feb 02, 2026, Lance Yang wrote:
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 37dc8465e0f5..6a5e47ee4eb6 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -856,6 +856,12 @@ static void __init kvm_guest_init(void)
>>   #ifdef CONFIG_SMP
>>   	if (pv_tlb_flush_supported()) {
>>   		pv_ops.mmu.flush_tlb_multi = kvm_flush_tlb_multi;
>> +		/*
>> +		 * KVM's flush implementation calls native_flush_tlb_multi(),
>> +		 * which sends real IPIs when INVLPGB is not available.
> 
> Not on all (virtual) CPUs.  The entire point of KVM's PV TLB flush is to elide
> the IPIs.  If a vCPU was scheduled out by the host, the guest sets a flag and
> relies on the host to flush the TLB on behalf of the guest prior to the next
> VM-Enter.

Ah, I see. Thanks for the correction!

KVM only sends IPIs to running vCPUs; preempted ones are left out of the 
mask
and flushed on VM-Enter. So the old comment was wrong ...

IIUC, we still set the flag to true because only running vCPUs can be in a
software/lockless walk, and they all get the IPI, so the flush is enough.

Does that match what you had in mind?


Thanks,
Lance

> 
> 	for_each_cpu(cpu, flushmask) {
> 		/*
> 		 * The local vCPU is never preempted, so we do not explicitly
> 		 * skip check for local vCPU - it will never be cleared from
> 		 * flushmask.
> 		 */
> 		src = &per_cpu(steal_time, cpu);
> 		state = READ_ONCE(src->preempted);
> 		if ((state & KVM_VCPU_PREEMPTED)) {
> 			if (try_cmpxchg(&src->preempted, &state,
> 					state | KVM_VCPU_FLUSH_TLB))
> 				__cpumask_clear_cpu(cpu, flushmask);  <=== removes CPU from the IPI set
> 		}
> 	}
> 
> 	native_flush_tlb_multi(flushmask, info);
> 
>> +		if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
>> +			pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast = true;
>>   		pr_info("KVM setup pv remote TLB flush\n");
>>   	}