Re: [PATCH v3 5/8] KVM: x86/mmu: Add a field to control memslot rmap allocation

[Ben Gardon]

On Thu, May 6, 2021 at 11:43 AM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> Add a field to control whether new memslots should have rmaps allocated
> for them. As of this change, it's not safe to skip allocating rmaps, so
> the field is always set to allocate rmaps. Future changes will make it
> safe to operate without rmaps, using the TDP MMU. Then further changes
> will allow the rmaps to be allocated lazily when needed for nested
> oprtation.
>
> No functional change expected.
>
> Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  8 ++++++++
>  arch/x86/kvm/mmu/mmu.c          |  2 ++
>  arch/x86/kvm/x86.c              | 18 +++++++++++++-----
>  3 files changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index ad22d4839bcc..00065f9bbc5e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1122,6 +1122,12 @@ struct kvm_arch {
>          */
>         spinlock_t tdp_mmu_pages_lock;
>  #endif /* CONFIG_X86_64 */
> +
> +       /*
> +        * If set, rmaps have been allocated for all memslots and should be
> +        * allocated for any newly created or modified memslots.
> +        */
> +       bool memslots_have_rmaps;
>  };
>
>  struct kvm_vm_stat {
> @@ -1853,4 +1859,6 @@ static inline int kvm_cpu_get_apicid(int mps_cpu)
>
>  int kvm_cpu_dirty_log_size(void);
>
> +inline bool kvm_memslots_have_rmaps(struct kvm *kvm);

Woops, this shouldn't be marked inline as it creates build problems
for the next patch with some configs.

> +
>  #endif /* _ASM_X86_KVM_HOST_H */
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 930ac8a7e7c9..8761b4925755 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5469,6 +5469,8 @@ void kvm_mmu_init_vm(struct kvm *kvm)
>
>         kvm_mmu_init_tdp_mmu(kvm);
>
> +       kvm->arch.memslots_have_rmaps = true;
> +
>         node->track_write = kvm_mmu_pte_write;
>         node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
>         kvm_page_track_register_notifier(kvm, node);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fc32a7dbe4c4..d7a40ce342cc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10868,7 +10868,13 @@ static int alloc_memslot_rmap(struct kvm_memory_slot *slot,
>         return -ENOMEM;
>  }
>
> -static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> +bool kvm_memslots_have_rmaps(struct kvm *kvm)
> +{
> +       return kvm->arch.memslots_have_rmaps;
> +}
> +
> +static int kvm_alloc_memslot_metadata(struct kvm *kvm,
> +                                     struct kvm_memory_slot *slot,
>                                       unsigned long npages)
>  {
>         int i;
> @@ -10881,9 +10887,11 @@ static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
>          */
>         memset(&slot->arch, 0, sizeof(slot->arch));
>
> -       r = alloc_memslot_rmap(slot, npages);
> -       if (r)
> -               return r;
> +       if (kvm_memslots_have_rmaps(kvm)) {
> +               r = alloc_memslot_rmap(slot, npages);
> +               if (r)
> +                       return r;
> +       }
>
>         for (i = 1; i < KVM_NR_PAGE_SIZES; ++i) {
>                 struct kvm_lpage_info *linfo;
> @@ -10954,7 +10962,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>                                 enum kvm_mr_change change)
>  {
>         if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
> -               return kvm_alloc_memslot_metadata(memslot,
> +               return kvm_alloc_memslot_metadata(kvm, memslot,
>                                                   mem->memory_size >> PAGE_SHIFT);
>         return 0;
>  }
> --
> 2.31.1.607.g51e8a6a459-goog
>