linux-kernel - Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES

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Date:   Thu, 18 Feb 2021 08:39:22 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>, "bp@...e.de" <bp@...e.de>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "srutherford@...gle.com" <srutherford@...gle.com>,
        "venu.busireddy@...cle.com" <venu.busireddy@...cle.com>,
        "Singh, Brijesh" <brijesh.singh@....com>
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@...gle.com> 
> 
> On Wed, Feb 17, 2021, Kalra, Ashish wrote:
> >> From: Sean Christopherson <seanjc@...gle.com> On Thu, Feb 04, 2021, 
> >> Ashish Kalra wrote:
> >> > From: Brijesh Singh <brijesh.singh@....com>
> >> > 
> >> > 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.