Re: [PATCH] x86/pvh: fix population of the low 1MB for dom0

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

On Mon, Jan 24, 2022 at 05:01:04PM +0100, Jan Beulich wrote:
> On 24.01.2022 16:23, Roger Pau Monne wrote:
> > RMRRs are setup ahead of populating the p2m and hence the ASSERT when
> > populating the low 1MB needs to be relaxed when it finds an existing
> > entry: it's either RAM or a RMRR resulting from the IOMMU setup.
> > 
> > Rework the logic a bit and introduce a local mfn variable in order to
> > assert that if the gfn is populated and not RAM it is an identity map.
> > 
> > Fixes: 6b4f6a31ac ('x86/PVH: de-duplicate mappings for first Mb of Dom0 
> > memory')
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit ...
> 
> > --- a/xen/arch/x86/hvm/dom0_build.c
> > +++ b/xen/arch/x86/hvm/dom0_build.c
> > @@ -466,11 +466,17 @@ static int __init pvh_populate_p2m(struct domain *d)
> >      for ( i = rc = 0; i < MB1_PAGES; ++i )
> >      {
> >          p2m_type_t p2mt;
> > +        mfn_t mfn;
> >  
> > -        if ( mfn_eq(get_gfn_query(d, i, &p2mt), INVALID_MFN) )
> > +        mfn = get_gfn_query(d, i, &p2mt);
> 
> ... preferably with this becoming the initializer of the variable then
> and ...
> 
> > +        if ( mfn_eq(mfn, INVALID_MFN) )
> >              rc = set_mmio_p2m_entry(d, _gfn(i), _mfn(i), PAGE_ORDER_4K);
> > -        else
> > -            ASSERT(p2mt == p2m_ram_rw);
> > +        else if ( p2mt != p2m_ram_rw && !mfn_eq(mfn, _mfn(i)) )
> > +                /*
> > +                 * If the p2m entry is already set it must belong to a 
> > RMRR and
> > +                 * already be identity mapped, or be a RAM region.
> > +                 */
> > +                ASSERT_UNREACHABLE();
> 
> ... (not just preferably) indentation reduced by a level here. (I wonder
> why you didn't simply extend the existing ASSERT() - ASSERT_UNREACHABLE()
> is nice in certain cases, but the expression it logs is entirely unhelpful
> for disambiguating the reason without looking at the source.)

Indeed, that's better. Sorry about the indentation, not sure what my
editor has done here.

Thanks, Roger.