Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

[Thomas HellstrÃm (VMware)]

On 9/5/19 4:15 PM, Dave Hansen wrote:
> Hi Thomas,
>
> Thanks for the second batch of patches!  These look much improved on all
> fronts.

Yes, although the TTM functionality isn't in yet. Hopefully we won't 
have to bother you with those though, since this assumes TTM will be 
using the dma API.

> On 9/5/19 3:35 AM, Thomas HellstrÃm (VMware) wrote:
> > -/* mprotect needs to preserve PAT bits when updating vm_page_prot */
> > +/*
> > + * mprotect needs to preserve PAT and encryption bits when updating
> > + * vm_page_prot
> > + */
> >   #define pgprot_modify pgprot_modify
> >   static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
> >   {
> > -	pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
> > -	pgprotval_t addbits = pgprot_val(newprot);
> > +	pgprotval_t preservebits = pgprot_val(oldprot) &
> > +		(_PAGE_CHG_MASK | sme_me_mask);
> > +	pgprotval_t addbits = pgprot_val(newprot) & ~sme_me_mask;
> >   	return __pgprot(preservebits | addbits);
> >   }
> _PAGE_CHG_MASK is claiming similar functionality about preserving bits
> when changing PTEs:
>
> > /*
> >   * Set of bits not changed in pte_modify.  The pte's
> >   * protection key is treated like _PAGE_RW, for
> >   * instance, and is *not* included in this mask since
> >   * pte_modify() does modify it.
> >   */
> > #define _PAGE_CHG_MASK  (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |         \
> >                           _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \
> >                           _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
> This makes me wonder if we should be including sme_me_mask in
> _PAGE_CHG_MASK (logically).

I was thinking the same. But what confuses me is that addbits isn't 
masked with ~_PAGE_CHG_MASK, which is needed for sme_me_mask, since the 
problem otherwise is typically that the encryption bit is incorrectly 
set in addbits. I wonder whether it's an optimization or intentional.

/Thomas