linux-kernel - Re: [PATCH v2 14/27] KVM: x86: Reject memslot MOVE operations if KVMGT is attached

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Date:   Thu, 16 Mar 2023 17:27:47 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <kvm@...r.kernel.org>, <intel-gfx@...ts.freedesktop.org>,
        <linux-kernel@...r.kernel.org>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        "Ben Gardon" <bgardon@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        <intel-gvt-dev@...ts.freedesktop.org>,
        Zhi Wang <zhi.a.wang@...el.com>
Subject: Re: [PATCH v2 14/27] KVM: x86: Reject memslot MOVE operations if
 KVMGT is attached

On Wed, Mar 15, 2023 at 08:43:54AM -0700, Sean Christopherson wrote:
> > So, in theory, the new GFNs are not write tracked though the old ones are.
> > 
> > Is that acceptable for the internal page-track user?
> 
> It works because KVM zaps all SPTEs when a memslot is moved, i.e. the fact that
Oh, yes!
And KVM will not shadow SPTEs for a invalid memslot, so there's no
problem.
Thanks~

> KVM loses the write-tracking counts is benign.  I suspect no VMM actually does
> does KVM_MR_MOVE in conjunction with shadow paging, but the ongoing maintenance
> cost of supporting KVM_MR_MOVE is quite low at this point, so trying to rip it
> out isn't worth the pain of having to deal with potential ABI breakage.
> 
> Though in hindsight I wish I had tried disallowed moving memslots instead of
> fixing the various bugs a few years back. :-(