Re: [PATCH v8 2/8] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

On Thu, Sep 29, 2022, Isaku Yamahata wrote:
> On Thu, Sep 15, 2022 at 10:29:07PM +0800,
> Chao Peng <chao.p.peng@linux.intel.com> wrote:
> > @@ -4645,14 +4672,20 @@ static long kvm_vm_ioctl(struct file *filp,
> >             break;
> >     }
> >     case KVM_SET_USER_MEMORY_REGION: {
> > -           struct kvm_userspace_memory_region kvm_userspace_mem;
> > +           struct kvm_user_mem_region mem;
> > +           unsigned long size = sizeof(struct kvm_userspace_memory_region);
> > +
> > +           kvm_sanity_check_user_mem_region_alias();
> >  
> >             r = -EFAULT;
> > -           if (copy_from_user(&kvm_userspace_mem, argp,
> > -                                           sizeof(kvm_userspace_mem)))
> > +           if (copy_from_user(&mem, argp, size);
> > +                   goto out;
> > +
> > +           r = -EINVAL;
> > +           if (mem.flags & KVM_MEM_PRIVATE)
> >                     goto out;
> 
> Nit:  It's better to check if padding is zero.  Maybe rename it to reserved.
> 
> +               if (mem.pad1 || memchr_inv(mem.pad2, 0, sizeof(mem.pad2)))
> +                       goto out;

No need, KVM has more or less settled on using flags instead "reserving" bytes.
E.g. if/when another fancy feature comes along, we'll add another KVM_MEM_XYZ
and only consume the relevant fields when the flag is set.  Reserving bytes
doesn't work very well because it assumes that '0' is an invalid value, e.g. if
the future expansion is for a non-private file descriptor, then we'd need a new
flag even if KVM reserved bytes since fd=0 is valid.

The only reason to bother with pad2[14] at this time is to avoid having to 
define
yet another struct if/when the struct needs to expand again.  The struct 
definition
will still need to be changed, but at least we won't end up with struct
kvm_userspace_memory_region_really_extended.