Re: [PATCH v7 09/14] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

On Wed, Aug 03, 2022, Chao Peng wrote:
> On Fri, Jul 29, 2022 at 07:51:29PM +0000, Sean Christopherson wrote:
> > On Wed, Jul 06, 2022, Chao Peng wrote:
> > > @@ -1332,9 +1332,18 @@ yet and must be cleared on entry.
> > >   __u64 userspace_addr; /* start of the userspace allocated memory */
> > >    };
> > >  
> > > +  struct kvm_userspace_memory_region_ext {
> > > + struct kvm_userspace_memory_region region;
> > > + __u64 private_offset;
> > > + __u32 private_fd;
> > > + __u32 pad1;
> > > + __u64 pad2[14];
> > > +};
> > > +
> > >    /* for kvm_memory_region::flags */
> > >    #define KVM_MEM_LOG_DIRTY_PAGES        (1UL << 0)
> > >    #define KVM_MEM_READONLY       (1UL << 1)
> > > +  #define KVM_MEM_PRIVATE                (1UL << 2)
> > 
> > Very belatedly following up on prior feedback...
> > 
> >   | I think a flag is still needed, the problem is private_fd can be safely
> >   | accessed only when this flag is set, e.g. without this flag, we can't
> >   | copy_from_user these new fields since they don't exist for previous
> >   | kvm_userspace_memory_region callers.
> > 
> > I forgot about that aspect of things.  We don't technically need a dedicated
> > PRIVATE flag to handle that, but it does seem to be the least awful 
> > soltuion.
> > We could either add a generic KVM_MEM_EXTENDED_REGION or an entirely new
> > ioctl(), e.g. KVM_SET_USER_MEMORY_REGION2, but in both approaches there's a 
> > decent
> > chance that we'll end up needed individual "this field is valid" flags 
> > anways.
> > 
> > E.g. if KVM requires pad1 and pad2 to be zero to carve out future 
> > extensions,
> > then we're right back here if some future extension needs to treat '0' as a 
> > legal
> > input.
> 
> I had such practice (always rejecting none-zero 'pad' value when
> introducing new user APIs) in other project previously, but I rarely
> see that in KVM.

Ya, KVM often uses flags to indicate the validity of a field specifically so 
that
KVM doesn't misinterpret a '0' from an older userspace as an intended value.