Subject: Re: [RFC PATCH v12 14/33] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

From: Sean Christopherson <[email-masked]>
Date: Wed, 20 Sep 2023 07:24:08 -0700

On Tue, Sep 19, 2023, Binbin Wu wrote:
> 
> 
> On 9/14/2023 9:55 AM, Sean Christopherson wrote:
> [...]
> > +
> > +static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
> > +				      pgoff_t end)
> > +{
> > +	struct kvm_memory_slot *slot;
> > +	struct kvm *kvm = gmem->kvm;
> > +	unsigned long index;
> > +	bool flush = false;
> > +
> > +	KVM_MMU_LOCK(kvm);
> > +
> > +	kvm_mmu_invalidate_begin(kvm);
> > +
> > +	xa_for_each_range(&gmem->bindings, index, slot, start, end - 1) {
> > +		pgoff_t pgoff = slot->gmem.pgoff;
> > +
> > +		struct kvm_gfn_range gfn_range = {
> > +			.start = slot->base_gfn + max(pgoff, start) - pgoff,
> > +			.end = slot->base_gfn + min(pgoff + slot->npages, end) - pgoff,
> > +			.slot = slot,
> > +			.may_block = true,
> > +		};
> > +
> > +		flush |= kvm_mmu_unmap_gfn_range(kvm, &gfn_range);
> > +	}
> > +
> > +	if (flush)
> > +		kvm_flush_remote_tlbs(kvm);
> > +
> > +	KVM_MMU_UNLOCK(kvm);
> > +}
> > +
> > +static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
> > +				    pgoff_t end)
> > +{
> > +	struct kvm *kvm = gmem->kvm;
> > +
> > +	KVM_MMU_LOCK(kvm);
> > +	if (xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT))
> > +		kvm_mmu_invalidate_end(kvm);
> kvm_mmu_invalidate_begin() is called unconditionally in
> kvm_gmem_invalidate_begin(),
> but kvm_mmu_invalidate_end() is not here.
> This makes the kvm_gmem_invalidate_{begin, end}() calls asymmetric.

Another ouch :-(

And there should be no need to acquire mmu_lock() unconditionally, the inode's
mutex protects the bindings, not mmu_lock.

I'll get a fix posted today.  I think KVM can also add a sanity check to detect
unresolved invalidations, e.g.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7ba1ab1832a9..2a2d18070856 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1381,8 +1381,13 @@ static void kvm_destroy_vm(struct kvm *kvm)
         * No threads can be waiting in kvm_swap_active_memslots() as the
         * last reference on KVM has been dropped, but freeing
         * memslots would deadlock without this manual intervention.
+        *
+        * If the count isn't unbalanced, i.e. KVM did NOT unregister between
+        * a start() and end(), then there shouldn't be any in-progress
+        * invalidations.
         */
        WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait));
+       WARN_ON(!kvm->mn_active_invalidate_count && kvm->mmu_invalidate_in_progress);
        kvm->mn_active_invalidate_count = 0;
 #else
        kvm_flush_shadow_all(kvm);

or an alternative style

	if (kvm->mn_active_invalidate_count)
		kvm->mn_active_invalidate_count = 0;
	else
		WARN_ON(kvm->mmu_invalidate_in_progress)

> > +	KVM_MMU_UNLOCK(kvm);
> > +}
> > +
> > +static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> > +{
> > +	struct list_head *gmem_list = &inode->i_mapping->private_list;
> > +	pgoff_t start = offset >> PAGE_SHIFT;
> > +	pgoff_t end = (offset + len) >> PAGE_SHIFT;
> > +	struct kvm_gmem *gmem;
> > +
> > +	/*
> > +	 * Bindings must stable across invalidation to ensure the start+end
> > +	 * are balanced.
> > +	 */
> > +	filemap_invalidate_lock(inode->i_mapping);
> > +
> > +	list_for_each_entry(gmem, gmem_list, entry) {
> > +		kvm_gmem_invalidate_begin(gmem, start, end);
> > +		kvm_gmem_invalidate_end(gmem, start, end);
> > +	}
> Why to loop for each gmem in gmem_list here?
> 
> IIUIC, offset is the offset according to the inode, it is only meaningful to
> the inode passed in, i.e, it is only meaningful to the gmem binding with the
> inode, not others.

The code is structured to allow for multiple gmem instances per inode.  This isn't
actually possible in the initial code base, but it's on the horizon[*].  I included
the list-based infrastructure in this initial series to ensure that guest_memfd
can actually support multiple files per inode, and to minimize the churn when the
"link" support comes along.

[*] https://lore.kernel.org/all/[email-masked]