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

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

On 2026/2/27 02:24, Sean Christopherson wrote:
> On Thu, Feb 26, 2026, Lance Yang wrote:
>> 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?
> 
> No, because from the guest kernel's perspective, the vCPU is running.  The kernel
> can't make any assumptions about what code the vCPU was executing when the vCPU
> was preempted by the host scheduler, i.e. it's entirely possible the vCPU is in
> a software/lockless walk.

Thanks a lot for setting me straight!

So any PV that has its own things and doesn't call
native_flush_tlb_multi() directly cannot be trusted to provide the IPI
guarantees we need.

So we should only set the flag for the native path, which truly calls
native_flush_tlb_multi() directly.


Have a great weekend,
Lance