Re: Question about the "TLBs and I-cache are private to each vCPU" guarantee with VTTBR_EL2.CnP

[Tangnianyao]

On 7/7/2026 14:36, Oliver Upton wrote:
> On Tue, Jul 07, 2026 at 10:41:35AM +0800, Tangnianyao wrote:
>> On 7/6/2026 23:33, Mark Rutland wrote:
>>> If *both* of the vCPUs set TTBRn_EL1.CnP, then surely that is
>>> indistinguishable from physical CPUs:
>>>
>>>     PE0(core0,smt0)         PE1(core0,smt1)
>>>     cpu0 va->pa0        
>>>                             cpu1 flush local tlb
>>>                             cpu1 modify desc to va->pa1
>>>     cpu0 hit *va->pa1*
>>>
>>> Mark. 
>>>
>>> .
>>>
>> Thanks for the clarification.
>> Stage-1 CnP determines whether Stage-1 translation entries may be shared
>> across vCPUs, and the hardware is responsible for enforcing the
>> architectural semantics.
>>
>> Given that, why does KVM still need to guarantee that TLBs are private to
>> each vCPU?
>>
>> Assuming VTTBR_EL2.CnP == 1:
>> If TTBRx_EL1.CnP == 1, the guest is responsible for ensuring that the
>> translations referenced by TTBRx_EL1 are shareable, as required by the
>> architecture.
>> If TTBRx_EL1.CnP == 0, the hardware must ensure that Stage-1 translations
>> are not shared, again according to the architectural definition.
>>
>> The reason I'm asking is the potential performance impact. In a scenario
>> where multiple vCPUs of the same VM are scheduled onto a single PE, this
>> TLB flush may prevent a vCPU from reusing its previously populated
>> translation entries, potentially increasing TLB misses.
> The problem is you can't infer the state of the TLB based on the current
> value of CnP at stage-1. CnP only applies to the current TTBR;
> nothing stops the guest from using a mix of CnP=0/1 for different TTBRs.
>
> KVM still needs to invalidate in the case that the PE retained a CnP=0
> TLB entry for a different stage-1 context than the one configured at
> the time of vcpu_load().
>
> Thanks,
> Oliver
> .
>

Thanks for the explanation. When the guest uses `Stage-1 CnP = 0`, kvm
does need the flush to maintain the "private to each vCPU" guarantee.

Thanks,
Nianyao Tang