Re: [PATCH V2 2/5] KVM: Add tlb remote flush callback in kvm_x86_ops.

[Tianyu Lan <Tianyu.Lan@microsoft.com>]

Hi Paolo:
	Thanks for review.

On 7/18/2018 8:01 PM, Paolo Bonzini wrote:
> On 09/07/2018 11:02, Tianyu Lan wrote:
>> +	/*
>> +	 * Call kvm_arch_hv_tlb_remote first and go back old way when
>> +	 * return failure.
>> +	 */
>> +	if (!kvm_arch_hv_flush_remote_tlb(kvm))
>> +		return;
>> +
>>   	/*
>>   	 * Read tlbs_dirty before setting KVM_REQ_TLB_FLUSH in
>>   	 * kvm_make_all_cpus_request.
>>   	 */
>> -	long dirty_count = smp_load_acquire(&kvm->tlbs_dirty);
>> +	dirty_count = smp_load_acquire(&kvm->tlbs_dirty);
> 
> Also, the call to kvm_arch_flush_remote_tlbs should not replace the
> whole function.  It should be something like:
> 
>          long dirty_count = smp_load_acquire(&kvm->tlbs_dirty);
> 
>          /*
>           * We want to publish modifications to the page tables before reading
>           * mode. Pairs with a memory barrier in arch-specific code.
>           * - x86: smp_mb__after_srcu_read_unlock in vcpu_enter_guest
>           * and smp_mb in walk_shadow_page_lockless_begin/end.
>           * - powerpc: smp_mb in kvmppc_prepare_to_enter.
>           *
>           * There is already an smp_mb__after_atomic() before
>           * kvm_make_all_cpus_request() reads vcpu->mode. We reuse that
>           * barrier here.
>           */
>          if (!kvm_arch_hv_flush_remote_tlb(kvm) ||
> 	    kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
>                  ++kvm->stat.remote_tlb_flush;
>          cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
> 
> Otherwise, kvm_mmu_notifier_invalidate_range_start will incorrectly skip
> a TLB flush.

Nice catch. Will update in the next version.

> 
> Thanks,
> 
> Paolo
>