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

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Date:   Wed, 17 Feb 2021 08:13:04 -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 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.

> >Also, aren't there plans for an in-guest migration helper?  If so, do we
> >have any idea what that interface will look like?  E.g. if we're going to
> >end up with a full >fledged driver in the guest, why not bite the bullet now
> >and bypass KVM entirely?
> 
> Even the in-guest migration helper will be using page encryption status
> hypercalls, so some interface is surely required.

If it's a driver with a more extensive interace, then the hypercalls can be
replaced by a driver operation.  That's obviously a big if, though.

> Also the in-guest migration will be mainly an OVMF component, won't  really
> be a full fledged kernel driver in the guest.

Is there code and/or a description of what the proposed helper would look like?