Re: [PATCH, RFC 49/62] mm, x86: export several MKTME variables

[Peter Zijlstra]

On Mon, Jun 17, 2019 at 03:14:29PM +1200, Kai Huang wrote:
> On Fri, 2019-06-14 at 13:56 +0200, Peter Zijlstra wrote:
> > On Wed, May 08, 2019 at 05:44:09PM +0300, Kirill A. Shutemov wrote:
> > > From: Kai Huang <kai.huang@xxxxxxxxxxxxxxx>
> > > 
> > > KVM needs those variables to get/set memory encryption mask.
> > > 
> > > Signed-off-by: Kai Huang <kai.huang@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > ---
> > >  arch/x86/mm/mktme.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/arch/x86/mm/mktme.c b/arch/x86/mm/mktme.c
> > > index df70651816a1..12f4266cf7ea 100644
> > > --- a/arch/x86/mm/mktme.c
> > > +++ b/arch/x86/mm/mktme.c
> > > @@ -7,13 +7,16 @@
> > >  
> > >  /* Mask to extract KeyID from physical address. */
> > >  phys_addr_t mktme_keyid_mask;
> > > +EXPORT_SYMBOL_GPL(mktme_keyid_mask);
> > >  /*
> > >   * Number of KeyIDs available for MKTME.
> > >   * Excludes KeyID-0 which used by TME. MKTME KeyIDs start from 1.
> > >   */
> > >  int mktme_nr_keyids;
> > > +EXPORT_SYMBOL_GPL(mktme_nr_keyids);
> > >  /* Shift of KeyID within physical address. */
> > >  int mktme_keyid_shift;
> > > +EXPORT_SYMBOL_GPL(mktme_keyid_shift);
> > >  
> > >  DEFINE_STATIC_KEY_FALSE(mktme_enabled_key);
> > >  EXPORT_SYMBOL_GPL(mktme_enabled_key);
> > 
> > NAK, don't export variables. Who owns the values, who enforces this?
> > 
> 
> Both KVM and IOMMU driver need page_keyid() and mktme_keyid_shift to set page's keyID to the right
> place in the PTE (of KVM EPT and VT-d DMA page table).
> 
> MKTME key type code need to know mktme_nr_keyids in order to alloc/free keyID.
> 
> Maybe better to introduce functions instead of exposing variables directly?
> 
> Or instead of introducing page_keyid(), we use page_encrypt_mask(), which essentially holds
> "page_keyid() << mktme_keyid_shift"?

Yes, that's much better, because that strictly limits the access to R/O.