linux-kernel - Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

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Date:   Fri, 24 Jun 2022 17:02:46 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Nikunj A. Dadhania" <nikunj@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        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>,
        Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com
Subject: Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

On Fri, Jun 24, 2022 at 09:28:23AM +0530, Nikunj A. Dadhania wrote:
> On 5/19/2022 9:07 PM, Chao Peng wrote:
> > A page fault can carry the information of whether the access if private
> > or not for KVM_MEM_PRIVATE memslot, this can be filled by architecture
> > code(like TDX code). To handle page faut for such access, KVM maps the
> > page only when this private property matches host's view on this page
> > which can be decided by checking whether the corresponding page is
> > populated in the private fd or not. A page is considered as private when
> > the page is populated in the private fd, otherwise it's shared.
> > 
> > For a successful match, private pfn is obtained with memfile_notifier
> > callbacks from private fd and shared pfn is obtained with existing
> > get_user_pages.
> > 
> > For a failed match, KVM causes a KVM_EXIT_MEMORY_FAULT exit to
> > userspace. Userspace then can convert memory between private/shared from
> > host's view then retry the access.
> > 
> > Co-developed-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> > Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> > Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> > ---
> >  arch/x86/kvm/mmu.h              |  1 +
> >  arch/x86/kvm/mmu/mmu.c          | 70 +++++++++++++++++++++++++++++++--
> >  arch/x86/kvm/mmu/mmu_internal.h | 17 ++++++++
> >  arch/x86/kvm/mmu/mmutrace.h     |  1 +
> >  arch/x86/kvm/mmu/paging_tmpl.h  |  5 ++-
> >  include/linux/kvm_host.h        | 22 +++++++++++
> >  6 files changed, 112 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > index 7e258cc94152..c84835762249 100644
> > --- a/arch/x86/kvm/mmu.h
> > +++ b/arch/x86/kvm/mmu.h
> > @@ -176,6 +176,7 @@ struct kvm_page_fault {
> >  
> >  	/* Derived from mmu and global state.  */
> >  	const bool is_tdp;
> > +	const bool is_private;
> >  	const bool nx_huge_page_workaround_enabled;
> >  
> >  	/*
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index afe18d70ece7..e18460e0d743 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -2899,6 +2899,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
> >  	if (max_level == PG_LEVEL_4K)
> >  		return PG_LEVEL_4K;
> >  
> > +	if (kvm_slot_is_private(slot))
> > +		return max_level;
> 
> Can you explain the rationale behind the above change? 
> AFAIU, this overrides the transparent_hugepage=never setting for both 
> shared and private mappings.

As Sean pointed out, this should check against fault->is_private instead
of the slot. For private fault, the level is retrieved and stored to
fault->max_level in kvm_faultin_pfn_private() instead of here.

For shared fault, it will continue to query host_level below. For
private fault, the host level has already been accounted in
kvm_faultin_pfn_private().

Chao
> 
> >  	host_level = host_pfn_mapping_level(kvm, gfn, pfn, slot);
> >  	return min(host_level, max_level);
> >  }
> 
> Regards
> Nikunj