Subject: Re: [RFC PATCH v11 06/29] KVM: Introduce KVM_SET_USER_MEMORY_REGION2

From: Sean Christopherson <[email-masked]>
Date: Fri, 28 Jul 2023 17:03:33 -0700

On Fri, Jul 28, 2023, Quentin Perret wrote:
> On Tuesday 18 Jul 2023 at 16:44:49 (-0700), Sean Christopherson wrote:
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -95,6 +95,16 @@ struct kvm_userspace_memory_region {
> >  	__u64 userspace_addr; /* start of the userspace allocated memory */
> >  };
> >  
> > +/* for KVM_SET_USER_MEMORY_REGION2 */
> > +struct kvm_userspace_memory_region2 {
> > +	__u32 slot;
> > +	__u32 flags;
> > +	__u64 guest_phys_addr;
> > +	__u64 memory_size;
> > +	__u64 userspace_addr;
> > +	__u64 pad[16];
> 
> Should we replace that pad[16] with:
> 
> 	__u64 size;
> 
> where 'size' is the size of the structure as seen by userspace? This is
> used in other UAPIs (see struct sched_attr for example) and is a bit
> more robust for future extensions (e.g. an 'old' kernel can correctly
> reject a newer version of the struct with additional fields it doesn't
> know about if that makes sense, etc).

"flags" serves that purpose, i.e. allows userspace to opt-in to having KVM actually
consume what is currently just padding.

The padding is there mainly to simplify kernel/KVM code, e.g. the number of bytes
that KVM needs to copy in is static.

But now that I think more on this, I don't know why we didn't just unconditionally
bump the size of kvm_userspace_memory_region.  We tried to play games with unions
and overlays, but that was a mess[*].

KVM would need to do multiple uaccess reads, but that's not a big deal.  Am I
missing something, or did past us just get too clever and miss the obvious solution?

[*] https://lkml.kernel.org/r/Y7xrtf9FCuYRYm1q%40google.com