On Mon, Jun 24, 2024 at 9:37 AM maobibo <maobibo@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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. Huacai > > Regards > Bibo Mao > > > > Huacai > > > >> > >> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx> > >> --- > >> 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 > >> > >
