Re: [RFCv2 15/16] KVM: Unmap protected pages from direct mapping

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Oct 23, 2020 at 03:37:12PM +0300, Mike Rapoport wrote:
> On Tue, Oct 20, 2020 at 09:18:58AM +0300, Kirill A. Shutemov wrote:
> > If the protected memory feature enabled, unmap guest memory from
> > kernel's direct mappings.
> > 
> > Migration and KSM is disabled for protected memory as it would require a
> > special treatment.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  include/linux/mm.h       |  3 +++
> >  mm/huge_memory.c         |  8 ++++++++
> >  mm/ksm.c                 |  2 ++
> >  mm/memory.c              | 12 ++++++++++++
> >  mm/rmap.c                |  4 ++++
> >  virt/lib/mem_protected.c | 21 +++++++++++++++++++++
> >  6 files changed, 50 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index ee274d27e764..74efc51e63f0 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -671,6 +671,9 @@ static inline bool vma_is_kvm_protected(struct vm_area_struct *vma)
> >  	return vma->vm_flags & VM_KVM_PROTECTED;
> >  }
> >  
> > +void kvm_map_page(struct page *page, int nr_pages);
> > +void kvm_unmap_page(struct page *page, int nr_pages);
> 
> This still does not seem right ;-)
> 
> And I still think that map/unmap primitives shoud be a part of the
> generic mm rather than exported by KVM.

Ya, and going a step further, I suspect it will be cleaner in the long run if
the kernel does not automatically map or unmap when converting between private
and shared/public memory.  Conversions will be rare in a well behaved guest, so
exiting to userspace and forcing userspace to do the unmap->map would not be a
performance bottleneck.  In theory, userspace could also maintain separate
pools for private vs. public mappings, though I doubt any VMM will do that in
practice.