Re: [PATCHv3 07/17] x86/mm: Preserve KeyID on pte_modify() and pgprot_modify()

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Wed, Jun 13, 2018 at 06:13:03PM +0000, Dave Hansen wrote:
> On 06/12/2018 07:39 AM, Kirill A. Shutemov wrote:
> > Encrypted VMA will have KeyID stored in vma->vm_page_prot. This way we
> 
> "An encrypted VMA..."
> 
> > don't need to do anything special to setup encrypted page table entries
> > and don't need to reserve space for KeyID in a VMA.
> > 
> > This patch changes _PAGE_CHG_MASK to include KeyID bits. Otherwise they
> > are going to be stripped from vm_page_prot on the first pgprot_modify().
> > 
> > Define PTE_PFN_MASK_MAX similar to PTE_PFN_MASK but based on
> > __PHYSICAL_MASK_SHIFT. This way we include whole range of bits
> > architecturally available for PFN without referencing physical_mask and
> > mktme_keyid_mask variables.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  arch/x86/include/asm/pgtable_types.h | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> > index 1e5a40673953..e8ebe760b88d 100644
> > --- a/arch/x86/include/asm/pgtable_types.h
> > +++ b/arch/x86/include/asm/pgtable_types.h
> > @@ -121,8 +121,13 @@
> >   * protection key is treated like _PAGE_RW, for
> >   * instance, and is *not* included in this mask since
> >   * pte_modify() does modify it.
> > + *
> > + * It includes full range of PFN bits regardless if they were claimed for KeyID
> > + * or not: we want to preserve KeyID on pte_modify() and pgprot_modify().
> >   */
> > -#define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |		\
> > +#define PTE_PFN_MASK_MAX \
> > +	(((signed long)PAGE_MASK) & ((1ULL << __PHYSICAL_MASK_SHIFT) - 1))
> 
> "signed long" is really unusual to see.  Was that intentional?

Yes. That's trick with sign-extension, borrowed from PHYSICAL_PAGE_MASK
definition. It helps on 32-bit with PAE properly expand the PAGE_MASK to
64-bit.

I'll add comment.

> > +#define _PAGE_CHG_MASK	(PTE_PFN_MASK_MAX | _PAGE_PCD | _PAGE_PWT |		\
> >  			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
> >  			 _PAGE_SOFT_DIRTY)
> >  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)
> 
> This makes me a bit nervous.  We have some places (here) where we
> pretend that the KeyID is part of the paddr and then other places like
> pte_pfn() where it's not.

Other option is to include KeyID mask into _PAGE_CHG_MASK. But it means
_PAGE_CHG_MASK would need to reference *two* variables: physical_mask and
mktme_keyid_mask. I mentioned this in the commit message.

This is more efficient way to achieve the same compile-time without
referencing any variables.

> Seems like something that will come back to bite us.

Any suggestions?

-- 
 Kirill A. Shutemov