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

[Jarkko Sakkinen]

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.
> 
> 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)
> 
> /* for KVM_SET_USER_MEMORY_REGION2 */
> struct kvm_user_mem_region2 {
> 	__u32 slot;
> 	__u32 flags;
> 	__u64 guest_phys_addr;
> 	__u64 memory_size;
> 	__u64 userspace_addr;
> 	__u64 restricted_offset;
> 	__u32 restricted_fd;
> 	__u32 pad1;
> 	__u64 pad2[14];
> }
> 
> And it's consistent with other KVM ioctls(), e.g. KVM_SET_CPUID2.
> 
> Regarding the userspace side of things, please include Vishal's selftests in v11,
> it's impossible to properly review the uAPI changes without seeing the userspace
> side of things.  I'm in the process of reviewing Vishal's v2[*], I'll try to
> massage it into a set of patches that you can incorporate into your series.
> 
> [*] https://lore.kernel.org/all/20221205232341.4131240-1-vannapurve@xxxxxxxxxx

+1

BR, Jarkko