Re: [PATCH v10 02/14] KVM: Cache as_id in kvm_memory_slot

[Peter Xu]

On Thu, Jul 02, 2020 at 04:08:49PM -0700, Sean Christopherson wrote:
> On Mon, Jun 01, 2020 at 07:59:45AM -0400, Peter Xu wrote:
> > Cache the address space ID just like the slot ID.  It will be used in
> > order to fill in the dirty ring entries.
> > 
> > Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >  include/linux/kvm_host.h | 1 +
> >  virt/kvm/kvm_main.c      | 1 +
> >  2 files changed, 2 insertions(+)
> > 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 01276e3d01b9..5e7bbaf7a36b 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -346,6 +346,7 @@ struct kvm_memory_slot {
> >  	unsigned long userspace_addr;
> >  	u32 flags;
> >  	short id;
> > +	u16 as_id;
> >  };
> >  
> >  static inline unsigned long kvm_dirty_bitmap_bytes(struct kvm_memory_slot *memslot)
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 74bdb7bf3295..ebdd98a30e82 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1243,6 +1243,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >  	if (!mem->memory_size)
> >  		return kvm_delete_memslot(kvm, mem, &old, as_id);
> 
> This technically needs to set as_id in the deleted memslot.  I highly doubt
> it will ever matter from a functionality perspective, but it'd be confusing
> to encounter a memslot whose as_id did not match that of its owner.

Yeah it shouldn't matter because as_id is directly passed in to look up the
pointer of kvm_memslots in kvm_delete_memslot, and memslot->as_id shouldn't be
further referenced.

I can add a comment above if this can clarify things a bit:

+	u16 as_id; /* cache of as_id; only valid if npages != 0 */

Thanks,

-- 
Peter Xu