Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 23, 2025, Ackerley Tng wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > On Wed, Oct 22, 2025, Ackerley Tng wrote:
> >> Ackerley Tng <ackerleytng@google.com> writes:
> >> 
> >> Found another issue with KVM_CAP_MEMORY_ATTRIBUTES2.
> >> 
> >> KVM_CAP_MEMORY_ATTRIBUTES2 was defined to do the same thing as
> >> KVM_CAP_MEMORY_ATTRIBUTES, but that's wrong since
> >> KVM_CAP_MEMORY_ATTRIBUTES2 should indicate the presence of
> >> KVM_SET_MEMORY_ATTRIBUTES2 and struct kvm_memory_attributes2.
> >
> > No?  If no attributes are supported, whether or not KVM_SET_MEMORY_ATTRIBUTES2
> > exists is largely irrelevant.
> 
> That's true.
> 
> > We can even provide the same -ENOTTY errno by
> > checking that _any_ attributes are supported, i.e. so that doing
> > KVM_SET_MEMORY_ATTRIBUTES2 on KVM without any support whatsoever fails in the
> > same way that KVM with code support but no attributes fails.
> 
> IIUC KVM_SET_MEMORY_ATTRIBUTES doesn't fail with -ENOTTY now when there
> are no valid attributes.
> 
> Even if there's no valid attributes (as in
> kvm_supported_mem_attributes() returns 0), it's possible to call
> KVM_SET_MEMORY_ATTRIBUTES with .attributes set to 0, which will be a
> no-op, but will return 0.
> 
> I think this is kind of correct behavior since .attributes = 0 is
> actually a valid expression for "I want this range to be shared", and
> for a VM that doesn't support private memory, it's a valid expression.
> 
> 
> The other way that there are "no attributes" would be if there are no
> /VM/ attributes, in which case KVM_SET_MEMORY_ATTRIBUTES, sent to as a
> vm ioctl, will return -ENOTTY.

Ya, this is what I was trying to say with "_any_ attributes are supported".  I.e.
by "any" I meant "any attributes in KVM for VMs vs. gmems", not "any attributes
for this specific VM/gmem instance".

> > In other words, I don't see why it can't do both.  Even if we can't massage the
> > right errno, I would much rather KVM_SET_MEMORY_ATTRIBUTES2 enumerate the set of
> 
> Did you mean KVM_CAP_MEMORY_ATTRIBUTES2 in the line above?

Doh, yes.