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

[Chao Peng]

On Tue, Mar 29, 2022 at 07:23:04PM +0000, Sean Christopherson wrote:
> On Thu, Mar 10, 2022, Chao Peng wrote:
> > 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.end = min(gfn_range.end, slot->base_gfn + 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);
> 
> This should check the result of kvm_unmap_gfn_range() and flush only if necessary.

Yep.

> 
> kvm->mmu_notifier_seq needs to be incremented, otherwise KVM will incorrectly
> install a SPTE if the mapping is zapped between retrieving the pfn in faultin and
> installing it after acquire mmu_lock.

Good catch.

Chao
> 
> 
> > +	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
> >