On Tue, Jan 10, 2023, Chao Peng wrote:

On Mon, Jan 09, 2023 at 07:32:05PM +0000, Sean Christopherson wrote:

quoted

On Fri, Jan 06, 2023, Chao Peng wrote:

quoted

On Thu, Jan 05, 2023 at 11:23:01AM +0000, Jarkko Sakkinen wrote:

quoted

On Fri, Dec 02, 2022 at 02:13:41PM +0800, Chao Peng wrote:

quoted

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.

quoted

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.