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

[Chao Peng <chao.p.peng@linux.intel.com>]

On Tue, Apr 19, 2022 at 03:43:56PM -0700, Vishal Annapurve wrote:
> 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);

Right. For start we don't really need care about the uppper bound
here (will check below), so this should be enough:
	gfn_range.start = slot->base_gfn + gfn_range.start;

> 
> > +       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);

This is correct.

Thanks,
Chao
> 
> > +       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
> >