Hi Marc, I just can't figure out why having the mmap lock is not needed to walk the userspace page tables. Any hints? Or am I not seeing where it's taken?
On 7/17/21 10:55 AM, Marc Zyngier wrote: > We currently rely on the kvm_is_transparent_hugepage() helper to > discover whether a given page has the potential to be mapped as > a block mapping. > > However, this API doesn't really give un everything we want: > - we don't get the size: this is not crucial today as we only > support PMD-sized THPs, but we'd like to have larger sizes > in the future > - we're the only user left of the API, and there is a will > to remove it altogether > > To address the above, implement a simple walker using the existing > page table infrastructure, and plumb it into transparent_hugepage_adjust(). > No new page sizes are supported in the process. > > Signed-off-by: Marc Zyngier <[email protected]> > --- > arch/arm64/kvm/mmu.c | 46 ++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 42 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > index 3155c9e778f0..db6314b93e99 100644 > --- a/arch/arm64/kvm/mmu.c > +++ b/arch/arm64/kvm/mmu.c > @@ -433,6 +433,44 @@ int create_hyp_exec_mappings(phys_addr_t phys_addr, > size_t size, > return 0; > } > > +static struct kvm_pgtable_mm_ops kvm_user_mm_ops = { > + /* We shouldn't need any other callback to walk the PT */ > + .phys_to_virt = kvm_host_va, > +}; > + > +struct user_walk_data { > + u32 level; > +}; > + > +static int user_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, > + enum kvm_pgtable_walk_flags flag, void * const arg) > +{ > + struct user_walk_data *data = arg; > + > + data->level = level; > + return 0; > +} > + > +static int get_user_mapping_size(struct kvm *kvm, u64 addr) > +{ > + struct user_walk_data data; > + struct kvm_pgtable pgt = { > + .pgd = (kvm_pte_t *)kvm->mm->pgd, > + .ia_bits = VA_BITS, > + .start_level = 4 - CONFIG_PGTABLE_LEVELS, > + .mm_ops = &kvm_user_mm_ops, > + }; > + struct kvm_pgtable_walker walker = { > + .cb = user_walker, > + .flags = KVM_PGTABLE_WALK_LEAF, > + .arg = &data, > + }; > + > + kvm_pgtable_walk(&pgt, ALIGN_DOWN(addr, PAGE_SIZE), PAGE_SIZE, &walker); I take it that it is guaranteed that kvm_pgtable_walk() will never fail? For example, I can see it failing if someone messes with KVM_PGTABLE_MAX_LEVELS. To be honest, I would rather have a check here instead of potentially feeding a bogus value to ARM64_HW_PGTABLE_LEVEL_SHIFT. It could be a VM_WARN_ON, so there's no runtime overhead unless CONFIG_DEBUG_VM. The patch looks good to me so far, but I want to give it another look (or two) after I figure out why the mmap semaphone is not needed. Thanks, Alex > + > + return BIT(ARM64_HW_PGTABLE_LEVEL_SHIFT(data.level)); > +} > + > static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = { > .zalloc_page = stage2_memcache_zalloc_page, > .zalloc_pages_exact = kvm_host_zalloc_pages_exact, > @@ -780,7 +818,7 @@ static bool fault_supports_stage2_huge_mapping(struct > kvm_memory_slot *memslot, > * Returns the size of the mapping. > */ > static unsigned long > -transparent_hugepage_adjust(struct kvm_memory_slot *memslot, > +transparent_hugepage_adjust(struct kvm *kvm, struct kvm_memory_slot *memslot, > unsigned long hva, kvm_pfn_t *pfnp, > phys_addr_t *ipap) > { > @@ -791,8 +829,8 @@ transparent_hugepage_adjust(struct kvm_memory_slot > *memslot, > * sure that the HVA and IPA are sufficiently aligned and that the > * block map is contained within the memslot. > */ > - if (kvm_is_transparent_hugepage(pfn) && > - fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) { > + if (fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE) && > + get_user_mapping_size(kvm, hva) >= PMD_SIZE) { > /* > * The address we faulted on is backed by a transparent huge > * page. However, because we map the compound huge page and > @@ -1051,7 +1089,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, > phys_addr_t fault_ipa, > * backed by a THP and thus use block mapping if possible. > */ > if (vma_pagesize == PAGE_SIZE && !(force_pte || device)) > - vma_pagesize = transparent_hugepage_adjust(memslot, hva, > + vma_pagesize = transparent_hugepage_adjust(kvm, memslot, hva, > &pfn, &fault_ipa); > > if (fault_status != FSC_PERM && !device && kvm_has_mte(kvm)) { _______________________________________________ kvmarm mailing list [email protected] https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
