Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES_LIST ioctl

[Sean Christopherson]

On Thu, Feb 18, 2021, Kalra, Ashish wrote:
> From: Sean Christopherson <seanjc@xxxxxxxxxx> 
> 
> On Wed, Feb 17, 2021, Kalra, Ashish wrote:
> >> From: Sean Christopherson <seanjc@xxxxxxxxxx> On Thu, Feb 04, 2021, 
> >> Ashish Kalra wrote:
> >> > From: Brijesh Singh <brijesh.singh@xxxxxxx>
> >> > 
> >> > The ioctl is used to retrieve a guest's shared pages list.
> >> 
> >> >What's the performance hit to boot time if KVM_HC_PAGE_ENC_STATUS is 
> >> >passed through to userspace?  That way, userspace could manage the 
> >> >set of pages >in whatever data structure they want, and these get/set ioctls go away.
> >> 
> >> What is the advantage of passing KVM_HC_PAGE_ENC_STATUS through to 
> >> user-space ?
> >> 
> >> As such it is just a simple interface to get the shared page list via 
> >> the get/set ioctl's. simply an array is passed to these ioctl to 
> >> get/set the shared pages list.
> 
> > It eliminates any probability of the kernel choosing the wrong data
> > structure, and it's two fewer ioctls to maintain and test.
> 
> The set shared pages list ioctl cannot be avoided as it needs to be issued to
> setup the shared pages list on the migrated VM, it cannot be achieved by
> passing KVM_HC_PAGE_ENC_STATUS through to user-space.

Why's that?  AIUI, KVM doesn't do anything with the list other than pass it back
to userspace.  Assuming that's the case, userspace can just hold onto the list
for the next migration.

> So it makes sense to add both get/set shared pages list ioctl, passing
> through to user-space is just adding more complexity without any significant
> gains.

No, it reduces complexity for KVM by avoiding locking and memslot dependencies.