Re: [PATCH] x86/shadow: replace p2m_is_valid() uses

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

On Thu, Mar 13, 2025 at 07:55:42AM +0100, Jan Beulich wrote:
> On 12.03.2025 13:30, Roger Pau Monné wrote:
> > On Wed, Mar 12, 2025 at 12:30:40PM +0100, Jan Beulich wrote:
> >> The justification for dropping p2m_mmio_dm from p2m_is_valid() was wrong
> >> for two of the shadow mode uses.
> >>
> >> In _sh_propagate() we want to create special L1 entries for p2m_mmio_dm
> >> pages. Hence we need to make sure we don't bail early for that type.
> >>
> >> In _sh_page_fault() we want to handle p2m_mmio_dm by forwarding to
> >> (internal or external) emulation. Pull the !p2m_is_mmio() check out of
> >> the || expression (as otherwise it would need adding to the lhs as
> >> well).
> >>
> >> In both cases, p2m_is_valid() in combination with p2m_is_grant() still
> >> doesn't cover foreign mappings. Hence use p2m_is_any_ram() plus (as
> >> necessary) p2m_mmio_* instead.
> >>
> >> Fixes: be59cceb2dbb ("x86/P2M: don't include MMIO_DM in p2m_is_valid()")
> >> Reported-by: Luca Fancellu <Luca.Fancellu@xxxxxxx>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > 
> > Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Thanks.
> 
> >> @@ -2366,8 +2371,8 @@ static int cf_check sh_page_fault(
> >>      gmfn = get_gfn(d, gfn, &p2mt);
> >>  
> >>      if ( shadow_mode_refcounts(d) &&
> >> -         ((!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt)) ||
> >> -          (!p2m_is_mmio(p2mt) && !mfn_valid(gmfn))) )
> >> +         !p2m_is_mmio(p2mt) &&
> >> +         (!p2m_is_any_ram(p2mt) || !mfn_valid(gmfn)) )
> > 
> > Would you mind adding some comment here about the need to forward
> > p2m_mmio_dm to the emulation, and hence not possible to short-circuit
> > here?
> 
> Will this do?
> 
>     /*
>      * p2m_mmio_dm in particular is handled further down, and hence can't be
>      * short-circuited here. Furthermore, while not fitting with architectural
>      * behavior, propagating #PF to the guest when a sensible shadow entry
>      * can't be written is necessary. Without doing so (by installing a non-
>      * present entry) we'd get back right here immediately afterwards, thus
>      * preventing the guest from making further forward progress.
>      */

LGTM.

Thanks, Roger.