linux-kernel - Re: [PATCH v3 kvm/queue 11/16] KVM: Add kvm_map_gfn

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Date:   Wed, 5 Jan 2022 17:03:23 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v3 kvm/queue 11/16] KVM: Add kvm_map_gfn_range

On Wed, Jan 05, 2022, Chao Peng wrote:
> On Tue, Jan 04, 2022 at 05:31:30PM +0000, Sean Christopherson wrote:
> > On Fri, Dec 31, 2021, Chao Peng wrote:
> > > On Fri, Dec 24, 2021 at 12:13:51PM +0800, Chao Peng wrote:
> > > > On Thu, Dec 23, 2021 at 06:06:19PM +0000, Sean Christopherson wrote:
> > > > > On Thu, Dec 23, 2021, Chao Peng wrote:
> > > > > > This new function establishes the mapping in KVM page tables for a
> > > > > > given gfn range. It can be used in the memory fallocate callback for
> > > > > > memfd based memory to establish the mapping for KVM secondary MMU when
> > > > > > the pages are allocated in the memory backend.
> > > > > 
> > > > > NAK, under no circumstance should KVM install SPTEs in response to allocating
> > > > > memory in a file.   The correct thing to do is to invalidate the gfn range
> > > > > associated with the newly mapped range, i.e. wipe out any shared SPTEs associated
> > > > > with the memslot.
> > > > 
> > > > Right, thanks.
> > > 
> > > BTW, I think the current fallocate() callback is just useless as long as
> > > we don't want to install KVM SPTEs in response to allocating memory in a
> > > file. The invalidation of the shared SPTEs should be notified through 
> > > mmu_notifier of the shared memory backend, not memfd_notifier of the
> > > private memory backend.
> > 
> > No, because the private fd is the final source of truth as to whether or not a
> > GPA is private, e.g. userspace may choose to not unmap the shared backing.
> > KVM's rule per Paolo's/this proposoal is that a GPA is private if it has a private
> > memslot and is present in the private backing store.  And the other core rule is
> > that KVM must never map both the private and shared variants of a GPA into the
> > guest.
> 
> That's true, but I'm wondering if zapping the shared variant can be
> handled at the time when the private one gets mapped in the KVM page
> fault. No bothering the backing store to dedicate a callback to tell
> KVM.

Hmm, I don't think that would work for the TDP MMU due to page faults taking
mmu_lock for read.  E.g. if two vCPUs concurrently fault in both the shared and
private variants, a race could exist where the private page fault sees the gfn
as private and the shared page fault sees it as shared.  In that case, both faults
will install a SPTE and KVM would end up running with both variants mapped into the
guest.

There's also a performance penalty, as KVM would need to walk the shared EPT tree
on every private page fault.