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

[Kalra, Ashish]

[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