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

[Chao Peng]

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@linux.intel.com>
> > Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> > Signed-off-by: Chao Peng <chao.p.peng@linux.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