Re: [RFC PATCH v2 5/5] KVM: Unmap pages only when it's indeed protected for NUMA migration

From: bibo mao <maobibo@loongson.cn>
To: Sean Christopherson <seanjc@google.com>, Yan Zhao <yan.y.zhao@intel.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, pbonzini@redhat.com,
	mike.kravetz@oracle.com, apopple@nvidia.com, jgg@nvidia.com,
	rppt@kernel.org, akpm@linux-foundation.org, kevin.tian@intel.com,
	david@redhat.com
Subject: Re: [RFC PATCH v2 5/5] KVM: Unmap pages only when it's indeed protected for NUMA migration
Date: Wed, 16 Aug 2023 10:43:44 +0800	[thread overview]
Message-ID: <107cdaaf-237f-16b9-ebe2-7eefd2b21f8f@loongson.cn> (raw)
In-Reply-To: <ZNuQ0grC44Dbh5hS@google.com>


在 2023/8/15 22:50, Sean Christopherson 写道:
> On Tue, Aug 15, 2023, Yan Zhao wrote:
>> On Mon, Aug 14, 2023 at 09:40:44AM -0700, Sean Christopherson wrote:
>>>>> Note, I'm assuming secondary MMUs aren't allowed to map swap entries...
>>>>>
>>>>> Compile tested only.
>>>>
>>>> I don't find a matching end to each
>>>> mmu_notifier_invalidate_range_start_nonblock().
>>>
>>> It pairs with existing call to mmu_notifier_invalidate_range_end() in change_pmd_range():
>>>
>>> 	if (range.start)
>>> 		mmu_notifier_invalidate_range_end(&range);
>> No, It doesn't work for mmu_notifier_invalidate_range_start() sent in change_pte_range(),
>> if we only want the range to include pages successfully set to PROT_NONE.
> 
> Precise invalidation was a non-goal for my hack-a-patch.  The intent was purely
> to defer invalidation until it was actually needed, but still perform only a
> single notification so as to batch the TLB flushes, e.g. the start() call still
> used the original @end.
> 
> The idea was to play nice with the scenario where nothing in a VMA could be migrated.
> It was complete untested though, so it may not have actually done anything to reduce
> the number of pointless invalidations.
For numa-balance scenery, can original page still be used by application even if pte
is changed with PROT_NONE?  If it can be used, maybe we can zap shadow mmu and flush tlb
in notification mmu_notifier_invalidate_range_end with precised range, the range can
be cross-range between range mmu_gather and mmu_notifier_range.

Regards
Bibo Mao
> 
>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>> index 9e4cd8b4a202..f29718a16211 100644
>>> --- a/arch/x86/kvm/mmu/mmu.c
>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>> @@ -4345,6 +4345,9 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>>>  	if (unlikely(!fault->slot))
>>>  		return kvm_handle_noslot_fault(vcpu, fault, access);
>>>  
>>> +	if (mmu_invalidate_retry_hva(vcpu->kvm, fault->mmu_seq, fault->hva))
>>> +		return RET_PF_RETRY;
>>> +
>> This can effectively reduce the remote flush IPIs a lot!
>> One Nit is that, maybe rmb() or READ_ONCE() is required for kvm->mmu_invalidate_range_start
>> and kvm->mmu_invalidate_range_end.
>> Otherwise, I'm somewhat worried about constant false positive and retry.
> 
> If anything, this needs a READ_ONCE() on mmu_invalidate_in_progress.  The ranges
> aren't touched when when mmu_invalidate_in_progress goes to zero, so ensuring they
> are reloaded wouldn't do anything.  The key to making forward progress is seeing
> that there is no in-progress invalidation.
> 
> I did consider adding said READ_ONCE(), but practically speaking, constant false
> positives are impossible.  KVM will re-enter the guest when retrying, and there
> is zero chance of the compiler avoiding reloads across VM-Enter+VM-Exit.
> 
> I suppose in theory we might someday differentiate between "retry because a different
> vCPU may have fixed the fault" and "retry because there's an in-progress invalidation",
> and not bother re-entering the guest for the latter, e.g. have it try to yield
> instead.  
> 
> All that said, READ_ONCE() on mmu_invalidate_in_progress should effectively be a
> nop, so it wouldn't hurt to be paranoid in this case.
> 
> Hmm, at that point, it probably makes sense to add a READ_ONCE() for mmu_invalidate_seq
> too, e.g. so that a sufficiently clever compiler doesn't completely optimize away
> the check.  Losing the check wouldn't be problematic (false negatives are fine,
> especially on that particular check), but the generated code would *look* buggy.