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 12:18:29 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: maobibo <maobibo@...ngson.cn>, Rui Wang <wangrui@...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

On Mon, Jun 24, 2024 at 10:21 AM maobibo <maobibo@...ngson.cn> wrote:
>
>
>
> 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?
There are some possibilities:
1. Read barrier is missing in some places;
2. Write barrier is also unnecessary here;
3. Read barrier is really unnecessary, but there is a better API to
replace the write barrier;
4. Read barrier is really unnecessary, and write barrier is really the
best API here.

Maybe Rui Wang knows better here.

Huacai

>
> 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
> >>>>
> >>
> >>
>
>