[AMD Public Use] -----Original Message----- From: Sean Christopherson <seanjc@xxxxxxxxxx> Sent: Thursday, February 18, 2021 10:39 AM To: Kalra, Ashish <Ashish.Kalra@xxxxxxx> Cc: pbonzini@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; hpa@xxxxxxxxx; rkrcmar@xxxxxxxxxx; joro@xxxxxxxxxx; bp@xxxxxxx; Lendacky, Thomas <Thomas.Lendacky@xxxxxxx>; x86@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; srutherford@xxxxxxxxxx; venu.busireddy@xxxxxxxxxx; Singh, Brijesh <brijesh.singh@xxxxxxx> 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 <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. 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
