Re: [PATCH 2/3] x86/P2M: correct old entry checking in p2m_remove_entry()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Mar 10, 2025 at 04:16:36PM +0100, Roger Pau Monné wrote:
> On Wed, Feb 26, 2025 at 12:52:45PM +0100, Jan Beulich wrote:
> > Using p2m_is_valid() isn't quite right here. It expanding to RAM+MMIO,
> > the subsequent p2m_mmio_direct check effectively reduces its use to
> > RAM+MMIO_DM. Yet MMIO_DM entries, which are never marked present in the
> > page tables, won't pass the mfn_valid() check. It is, however, quite
> > plausible (and supported by the rest of the function) to permit
> > "removing" hole entries, i.e. in particular to convert MMIO_DM to
> > INVALID. Which leaves the original check to be against RAM (plus MFN
> > validity), while HOLE then instead wants INVALID_MFN to be passed in.
> > 
> > Further more grant and foreign entries (together with RAM becoming
> > ANY_RAM) as well as BROKEN want the MFN checking, too.
> > 
> > All other types (i.e. MMIO_DIRECT and POD) want rejecting here rather
> > than skipping, for needing handling / accounting elsewhere.
> > 
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > ---
> > Paging/sharing types likely need further customization here. Paths
> > leading here differ in their handling (e.g. guest_remove_page() special-
> > casing paging types vs XENMEM_remove_from_physmap not doing so), so it's
> > not even clear what the intentions are for those types.
> > 
> > --- a/xen/arch/x86/mm/p2m.c
> > +++ b/xen/arch/x86/mm/p2m.c
> > @@ -531,9 +531,9 @@ p2m_remove_entry(struct p2m_domain *p2m,
> >          mfn_t mfn_return = p2m->get_entry(p2m, gfn_add(gfn, i), &t, &a, 0,
> >                                            &cur_order, NULL);
> >  
> > -        if ( p2m_is_valid(t) &&
> > -             (!mfn_valid(mfn) || t == p2m_mmio_direct ||
> > -              !mfn_eq(mfn_add(mfn, i), mfn_return)) )
> > +        if ( p2m_is_any_ram(t) || p2m_is_broken(t)
> > +             ? !mfn_valid(mfn) || !mfn_eq(mfn_add(mfn, i), mfn_return)
> 
> In the commit message you mention that MMIO_DIRECT wants rejecting
> here, yet I think some MMIO_DIRECT mfns can be valid IIRC, and hence
> would satisfy the check here?

Never mind, p2m_is_any_ram() doesn't allow MMIO_DIRECT, and hence
won't get into the mfn_valid() check in the first place.  I was
getting confused with the previous p2m_is_valid().

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.