Re: [PATCH 1/5] mmu: mark spte present if the x bit is set

[Xiao Guangrong]

On 06/29/2016 04:49 AM, Paolo Bonzini wrote:
> On 28/06/2016 22:37, Bandan Das wrote:
> > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> >
> > > On 28/06/2016 19:33, Bandan Das wrote:
> > > > > > > >   static int is_shadow_present_pte(u64 pte)
> > > > > > > >   {
> > > > > > > > -	return pte & PT_PRESENT_MASK && !is_mmio_spte(pte);
> > > > > > > > +	return pte & (PT_PRESENT_MASK | shadow_x_mask) &&
> > > > > > > > +		!is_mmio_spte(pte);
> > > > > >
> > > > > > This should really be pte & 7 when using EPT.  But this is okay as an
> > > > > > alternative to a new shadow_present_mask.
> > > > I could revive shadow_xonly_valid probably... Anyway, for now I will
> > > > add a TODO comment here.
> > >
> > > It's okay to it like this, because the only invalid PTEs reaching this
> > > point are those that is_mmio_spte filters away.  Hence you'll never get
> > > -W- PTEs here, and pte & 7 is really the same as how you wrote it.  It's
> > > pretty clever, and doesn't need a TODO at all. :)
> >
> > Thanks, understood. So, the way it is written now covers all cases for
> > pte & 7. Let's still add a comment - clever things are usually
> > confusing to many!
>
> I think another way to write it is "(pte & 0xFFFFFFFFull) &&
> !is_mmio_spte(pte)", since non-present/non-MMIO SPTEs never use bits
> 1..31 (they can have non-zero bits 32..63 on 32-bit CPUs where we don't
> update the PTEs atomically).  Guangrong, what do you prefer?

I think the way you innovated is better. :)