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

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Date: Sun, 23 Jun 2024 18:18:27 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
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

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.

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
>