Re: [PATCH v5 14/19] KVM: Clean up local variable usage in __kvm_set_memory_region()

Sean Christopherson
2020-02-06T19:22:30Z

On Thu, Feb 06, 2020 at 02:06:41PM -0500, Peter Xu wrote:
> On Tue, Jan 21, 2020 at 02:31:52PM -0800, Sean Christopherson wrote:
>
> [...]
>
> > @@ -1101,52 +1099,55 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
> > return -EINVAL;
> >
> > - slot = id_to_memslot(__kvm_memslots(kvm, as_id), id);
> > - base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
> > - npages = mem->memory_size >> PAGE_SHIFT;
> > -
> > - if (npages > KVM_MEM_MAX_NR_PAGES)
> > - return -EINVAL;
> > -
> > /*
> > * Make a full copy of the old memslot, the pointer will become stale
> > * when the memslots are re-sorted by update_memslots().
> > */
> > - old = *slot;
> > + tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
> > + old = *tmp;
> > + tmp = NULL;
>
> Shall we keep this chunk to the patch where it will be used? Other
> than that, it looks good to me.

I assume you're talking about doing this instead of using @tmp?

old = *id_to_memslot(__kvm_memslots(kvm, as_id), id);

It's obviously possible, but I really like resulting diff for
__kvm_set_memory_region() in "KVM: Terminate memslot walks via used_slots"
when tmp is used.

@@ -1104,8 +1203,13 @@ int __kvm_set_memory_region(struct kvm *kvm,
* when the memslots are re-sorted by update_memslots().
*/
tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
- old = *tmp;
- tmp = NULL;
+ if (tmp) {
+ old = *tmp;
+ tmp = NULL;
+ } else {
+ memset(&old, 0, sizeof(old));
+ old.id = id;
+ }