linux-kernel - Re: [PATCH v2 4/6] LoongArch: KVM: Add memory barrier before update pmd entry

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Date: Mon, 24 Jun 2024 10:21:42 +0800
From: maobibo <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Tianrui Zhao <zhaotianrui@...ngson.cn>, WANG Xuerui <kernel@...0n.name>,
 Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] LoongArch: KVM: Add memory barrier before update
 pmd entry



On 2024/6/24 上午9:56, Huacai Chen wrote:
> On Mon, Jun 24, 2024 at 9:37 AM maobibo <maobibo@...ngson.cn> wrote:
>>
>>
>>
>> On 2024/6/23 下午6:18, Huacai Chen wrote:
>>> Hi, Bibo,
>>>
>>> On Wed, Jun 19, 2024 at 4:09 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>>>>
>>>> When updating pmd entry such as allocating new pmd page or splitting
>>>> huge page into normal page, it is necessary to firstly update all pte
>>>> entries, and then update pmd entry.
>>>>
>>>> It is weak order with LoongArch system, there will be problem if other
>>>> vcpus sees pmd update firstly however pte is not updated. Here smp_wmb()
>>>> is added to assure this.
>>> Memory barriers should be in pairs in most cases. That means you may
>>> lose smp_rmb() in another place.
>> The idea adding smp_wmb() comes from function __split_huge_pmd_locked()
>> in file mm/huge_memory.c, and the explanation is reasonable.
>>
>>                   ...
>>                   set_ptes(mm, haddr, pte, entry, HPAGE_PMD_NR);
>>           }
>>           ...
>>           smp_wmb(); /* make pte visible before pmd */
>>           pmd_populate(mm, pmd, pgtable);
>>
>> It is strange that why smp_rmb() should be in pairs with smp_wmb(),
>> I never hear this rule -:(
> https://docs.kernel.org/core-api/wrappers/memory-barriers.html
> 
> SMP BARRIER PAIRING
> -------------------
> 
> When dealing with CPU-CPU interactions, certain types of memory barrier should
> always be paired.  A lack of appropriate pairing is almost certainly an error.
    CPU 1                 CPU 2
         ===============       ===============
         WRITE_ONCE(a, 1);
         <write barrier>
         WRITE_ONCE(b, 2);     x = READ_ONCE(b);
                               <read barrier>
                               y = READ_ONCE(a);

With split_huge scenery to update pte/pmd entry, there is no strong 
relationship between address ptex and pmd.
CPU1
      WRITE_ONCE(pte0, 1);
      WRITE_ONCE(pte511, 1);
      <write barrier>
      WRITE_ONCE(pmd, 2);

However with page table walk scenery, address ptep depends on the 
contents of pmd, so it is not necessary to add smp_rmb().
         ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
         if (!ptep)
                 return no_page_table(vma, flags, address);
         pte = ptep_get(ptep);
         if (!pte_present(pte))

It is just my option, or do you think where smp_rmb() barrier should be 
added in page table reader path?

Regards
Bibo Mao
> 
> 
> Huacai
> 
>>
>> Regards
>> Bibo Mao
>>>
>>> Huacai
>>>
>>>>
>>>> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
>>>> ---
>>>>    arch/loongarch/kvm/mmu.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
>>>> index 1690828bd44b..7f04edfbe428 100644
>>>> --- a/arch/loongarch/kvm/mmu.c
>>>> +++ b/arch/loongarch/kvm/mmu.c
>>>> @@ -163,6 +163,7 @@ static kvm_pte_t *kvm_populate_gpa(struct kvm *kvm,
>>>>
>>>>                           child = kvm_mmu_memory_cache_alloc(cache);
>>>>                           _kvm_pte_init(child, ctx.invalid_ptes[ctx.level - 1]);
>>>> +                       smp_wmb(); /* make pte visible before pmd */
>>>>                           kvm_set_pte(entry, __pa(child));
>>>>                   } else if (kvm_pte_huge(*entry)) {
>>>>                           return entry;
>>>> @@ -746,6 +747,7 @@ static kvm_pte_t *kvm_split_huge(struct kvm_vcpu *vcpu, kvm_pte_t *ptep, gfn_t g
>>>>                   val += PAGE_SIZE;
>>>>           }
>>>>
>>>> +       smp_wmb();
>>>>           /* The later kvm_flush_tlb_gpa() will flush hugepage tlb */
>>>>           kvm_set_pte(ptep, __pa(child));
>>>>
>>>> --
>>>> 2.39.3
>>>>
>>
>>