On Fri, Feb 07, 2020 at 04:46:23PM -0500, Peter Xu wrote:

On Fri, Feb 07, 2020 at 01:10:16PM -0800, Sean Christopherson wrote:

quoted

On Fri, Feb 07, 2020 at 03:39:09PM -0500, Peter Xu wrote:

quoted

On Fri, Feb 07, 2020 at 10:33:25AM -0800, Sean Christopherson wrote:

quoted

On Thu, Feb 06, 2020 at 04:09:44PM -0500, Peter Xu wrote:

quoted

On Tue, Jan 21, 2020 at 02:31:55PM -0800, Sean Christopherson wrote:

quoted

@@ -9652,13 +9652,13 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
 		if (IS_ERR((void *)hva))
 			return PTR_ERR((void *)hva);
 	} else {
-		if (!slot->npages)
+		if (!slot || !slot->npages)
 			return 0;
 
-		hva = 0;
+		hva = slot->userspace_addr;

Is this intended?

Yes.  It's possible to allow VA=0 for userspace mappings.  It's extremely
uncommon, but possible.  Therefore "hva == 0" shouldn't be used to
indicate an invalid slot.

Note that this is the deletion path in __x86_set_memory_region() not
allocation.  IIUC userspace_addr won't even be used in follow up code
path so it shouldn't really matter.  Or am I misunderstood somewhere?

No, but that's precisely why I don't want to zero out @hva, as doing so
implies that '0' indicates an invalid hva, which is wrong.

What if I change this to 

			hva = 0xdeadull << 48;

and add a blurb in the changelog about stuff hva with a non-canonical value
to indicate it's being destroyed.

IMO it's fairly common to have the case where "when A is XXX then
parameters B is invalid" happens in C.

I'm not arguing that's not the case.  My point is that there's nothing
special about '0', so why use it?  E.g. "hva = 1" would also be ok from a
functional perspective, but more obviously "wrong".

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