Re: [Xen-devel] [PATCH for-4.12 2/8] amd/ntp: remove assert that prevents creating 2M MMIO entries

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

On Tue, Feb 05, 2019 at 12:45:56AM -0700, Jan Beulich wrote:
> >>> On 04.02.19 at 18:18, <roger.pau@xxxxxxxxxx> wrote:
> > On Mon, Feb 04, 2019 at 09:56:22AM -0700, Jan Beulich wrote:
> >> >>> On 30.01.19 at 11:36, <roger.pau@xxxxxxxxxx> wrote:
> >> > The assert was originally added to make sure that higher order
> >> > regions (> PAGE_ORDER_4K) could not be used to bypass the
> >> > mmio_ro_ranges check performed by p2m_type_to_flags.
> >> > 
> >> > This however is already checked in set_mmio_p2m_entry, which makes
> >> > sure that higher order mappings don't overlap with mmio_ro_ranges,
> >> > thus allowing the creation of high order MMIO mappings safely.
> >> 
> >> Well, the assertions were added to make sure no other code
> >> path appears that violates this requirement. Arguably e.g.
> >> set_identity_p2m_entry() could gain an order parameter and
> >> then try to establish larger p2m_mmio_direct entries.
> >> 
> >> Don't get me wrong, I don't object to the removal of the
> >> assertions, but the description makes it sound as if they were
> >> entirely redundant. Even better would be though if they
> >> could be extended to keep triggering in "bad" cases.
> > 
> > I could add something like:
> > 
> > ASSERT(!rangeset_overlaps_range(mmio_ro_ranges, mfn_x(mfn),
> >                                 mfn_x(mfn) + PFN_DOWN(MB(2))));
> > 
> > I think this should be safe and would trigger in case of misuse.
> 
> Looks okay, if slightly extended (or made conditional) to exclude
> the addition of MB(2) to MFN_INVALID to wrap and potentially
> hit a r/o range in the low 1Mb.

Ack, so it would be:

ASSERT(mfn_eq(mfn, INVALID_MFN) ||
       !rangeset_overlaps_range(mmio_ro_ranges, mfn_x(mfn),
                                mfn_x(mfn) + PFN_DOWN(MB(2))));


> >> > --- a/xen/arch/x86/mm/p2m-pt.c
> >> > +++ b/xen/arch/x86/mm/p2m-pt.c
> >> > @@ -668,7 +668,6 @@ p2m_pt_set_entry(struct p2m_domain *p2m, gfn_t gfn_, 
> >> > mfn_t mfn,
> >> >          }
> >> >  
> >> >          ASSERT(p2m_flags_to_type(flags) != p2m_ioreq_server);
> >> > -        ASSERT(!mfn_valid(mfn) || p2mt != p2m_mmio_direct);
> >> >          l2e_content = mfn_valid(mfn) || p2m_allows_invalid_mfn(p2mt)
> >> >              ? p2m_l2e_from_pfn(mfn_x(mfn),
> >> >                                 p2m_type_to_flags(p2m, p2mt, mfn, 1))
> >> 
> >> There's a similar check in the 1G mapping logic several lines up. Why
> >> does that not also need removing?
> > 
> > So far mmio_order doesn't allow creation of 1G entries for mmio
> > regions, that's why I haven't removed that assert. I can however
> > replace it with the assert suggested above properly adjusted for 1G
> > pages.
> 
> Yes, this or explicitly say in the description why you leave alone the
> other one.

I think changing the assert is likely more future proof.

Thanks, Roger.

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