Re: [PATCH v5 11/13] KVM: Zap existing KVM mappings when pages changed in the private fd

[Vishal Annapurve]

On Thu, Mar 10, 2022 at 6:11 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
>
> KVM gets notified when memory pages changed in the memory backing store.
> When userspace allocates the memory with fallocate() or frees memory
> with fallocate(FALLOC_FL_PUNCH_HOLE), memory backing store calls into
> KVM fallocate/invalidate callbacks respectively. To ensure KVM never
> maps both the private and shared variants of a GPA into the guest, in
> the fallocate callback, we should zap the existing shared mapping and
> in the invalidate callback we should zap the existing private mapping.
>
> In the callbacks, KVM firstly converts the offset range into the
> gfn_range and then calls existing kvm_unmap_gfn_range() which will zap
> the shared or private mapping. Both callbacks pass in a memslot
> reference but we need 'kvm' so add a reference in memslot structure.
>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/linux/kvm_host.h |  3 ++-
>  virt/kvm/kvm_main.c      | 36 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9b175aeca63f..186b9b981a65 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -236,7 +236,7 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t 
> cr2_or_gpa,
>  int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu);
>  #endif
>
> -#ifdef KVM_ARCH_WANT_MMU_NOTIFIER
> +#if defined(KVM_ARCH_WANT_MMU_NOTIFIER) || defined(CONFIG_MEMFILE_NOTIFIER)
>  struct kvm_gfn_range {
>         struct kvm_memory_slot *slot;
>         gfn_t start;
> @@ -568,6 +568,7 @@ struct kvm_memory_slot {
>         loff_t private_offset;
>         struct memfile_pfn_ops *pfn_ops;
>         struct memfile_notifier notifier;
> +       struct kvm *kvm;
>  };
>
>  static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 67349421eae3..52319f49d58a 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -841,8 +841,43 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
>  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
>
>  #ifdef CONFIG_MEMFILE_NOTIFIER
> +static void kvm_memfile_notifier_handler(struct memfile_notifier *notifier,
> +                                        pgoff_t start, pgoff_t end)
> +{
> +       int idx;
> +       struct kvm_memory_slot *slot = container_of(notifier,
> +                                                   struct kvm_memory_slot,
> +                                                   notifier);
> +       struct kvm_gfn_range gfn_range = {
> +               .slot           = slot,
> +               .start          = start - (slot->private_offset >> 
> PAGE_SHIFT),
> +               .end            = end - (slot->private_offset >> PAGE_SHIFT),
> +               .may_block      = true,
> +       };
> +       struct kvm *kvm = slot->kvm;
> +
> +       gfn_range.start = max(gfn_range.start, slot->base_gfn);

gfn_range.start seems to be page offset within the file. Should this rather be:
gfn_range.start = slot->base_gfn + min(gfn_range.start, slot->npages);

> +       gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> +

Similar to previous comment, should this rather be:
gfn_range.end = slot->base_gfn + min(gfn_range.end, slot->npages);

> +       if (gfn_range.start >= gfn_range.end)
> +               return;
> +
> +       idx = srcu_read_lock(&kvm->srcu);
> +       KVM_MMU_LOCK(kvm);
> +       kvm_unmap_gfn_range(kvm, &gfn_range);
> +       kvm_flush_remote_tlbs(kvm);
> +       KVM_MMU_UNLOCK(kvm);
> +       srcu_read_unlock(&kvm->srcu, idx);
> +}
> +
> +static struct memfile_notifier_ops kvm_memfile_notifier_ops = {
> +       .invalidate = kvm_memfile_notifier_handler,
> +       .fallocate = kvm_memfile_notifier_handler,
> +};
> +
>  static inline int kvm_memfile_register(struct kvm_memory_slot *slot)
>  {
> +       slot->notifier.ops = &kvm_memfile_notifier_ops;
>         return memfile_register_notifier(file_inode(slot->private_file),
>                                          &slot->notifier,
>                                          &slot->pfn_ops);
> @@ -1963,6 +1998,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>         new->private_file = file;
>         new->private_offset = mem->flags & KVM_MEM_PRIVATE ?
>                               region_ext->private_offset : 0;
> +       new->kvm = kvm;
>
>         r = kvm_set_memslot(kvm, old, new, change);
>         if (!r)
> --
> 2.17.1
>