Re: [PATCH v10 3/9] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

On Tue, Jan 10, 2023, Chao Peng wrote:
> On Mon, Jan 09, 2023 at 07:32:05PM +0000, Sean Christopherson wrote:
> > On Fri, Jan 06, 2023, Chao Peng wrote:
> > > On Thu, Jan 05, 2023 at 11:23:01AM +0000, Jarkko Sakkinen wrote:
> > > > On Fri, Dec 02, 2022 at 02:13:41PM +0800, Chao Peng wrote:
> > > > > To make future maintenance easy, internally use a binary compatible
> > > > > alias struct kvm_user_mem_region to handle both the normal and the
> > > > > '_ext' variants.
> > > > 
> > > > Feels bit hacky IMHO, and more like a completely new feature than
> > > > an extension.
> > > > 
> > > > Why not just add a new ioctl? The commit message does not address
> > > > the most essential design here.
> > > 
> > > Yes, people can always choose to add a new ioctl for this kind of change
> > > and the balance point here is we want to also avoid 'too many ioctls' if
> > > the functionalities are similar.  The '_ext' variant reuses all the
> > > existing fields in the 'normal' variant and most importantly KVM
> > > internally can reuse most of the code. I certainly can add some words in
> > > the commit message to explain this design choice.
> > 
> > After seeing the userspace side of this, I agree with Jarkko; overloading
> > KVM_SET_USER_MEMORY_REGION is a hack.  E.g. the size validation ends up 
> > being
> > bogus, and userspace ends up abusing unions or implementing 
> > kvm_user_mem_region
> > itself.
> 
> How is the size validation being bogus? I don't quite follow.

The ioctl() magic embeds the size of the payload (struct 
kvm_userspace_memory_region
in this case) in the ioctl() number, and that information is visible to 
userspace
via _IOCTL_SIZE().  Attempting to take a larger size can mess up sanity checks,
e.g. KVM selftests get tripped up on this assert if KVM_SET_USER_MEMORY_REGION 
is
passed an "extended" struct.

        #define kvm_do_ioctl(fd, cmd, arg)                                      
        \
        ({                                                                      
        \
                kvm_static_assert(!_IOC_SIZE(cmd) || sizeof(*arg) == 
_IOC_SIZE(cmd));   \
                ioctl(fd, cmd, arg);                                            
        \
        })

> Then we will use kvm_userspace_memory_region2 as the KVM internal alias,
> right?

Yep.

> I see similar examples use different functions to handle different versions
> but it does look easier if we use alias for this function.
> 
> > 
> > It feels absolutely ridiculous, but I think the best option is to do:
> > 
> > #define KVM_SET_USER_MEMORY_REGION2 _IOW(KVMIO, 0x49, \
> >                                      struct kvm_userspace_memory_region2)
> 
> Just interesting, is 0x49 a safe number we can use? 

Yes?  So long as its not used by KVM, it's safe.  AFAICT, it's unused.