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 09:37:13 +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 2024/6/23 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 -:(

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
>>