[AMD Public Use]
-----Original Message----- From: Sean Christopherson <[email protected]> Sent: Thursday, February 18, 2021 10:39 AM To: Kalra, Ashish <[email protected]> Cc: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; Lendacky, Thomas <[email protected]>; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; Singh, Brijesh <[email protected]> Subject: Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES_LIST ioctl On Thu, Feb 18, 2021, Kalra, Ashish wrote: > From: Sean Christopherson <[email protected]> > > On Wed, Feb 17, 2021, Kalra, Ashish wrote: > >> From: Sean Christopherson <[email protected]> On Thu, Feb 04, 2021, > >> Ashish Kalra wrote: > >> > From: Brijesh Singh <[email protected]> > >> > > >> > 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. KVM does use it as part of the SEV DBG_DECTYPT API, within sev_dbg_decrypt() to check if the guest page(s) are encrypted or not, and accordingly use it to decide whether to decrypt the guest page(s) and return that back to user-space or just return it as it is. Thanks, Ashish
