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

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

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>

One suggestion below.

> ---
> This still leaves the p2m_mmio_dm vs p2m_invalid unaddressed.
> 
> While propagating #PF to the guest based on P2M type isn't quite right,
> not doing so in sh_page_fault() would lead to no forward progress in the
> guest anymore: If we put in place a non-present shadow PTE, another #PF
> will occur right after exiting to the guest. Doing so is only okay as a
> transient measure, e.g. while paging back in a paged-out page (where the
> respective type is included in P2M_RAM_TYPES).
> 
> There could likely be at least one more Fixes: tag (to cover the lack of
> handling foreign mappings); the one supplied is merely to indicate the
> connection to the recent regression.
> 
> --- a/xen/arch/x86/mm/shadow/multi.c
> +++ b/xen/arch/x86/mm/shadow/multi.c
> @@ -471,9 +471,7 @@ _sh_propagate(struct vcpu *v,
>      /* We don't shadow PAE l3s */
>      ASSERT(GUEST_PAGING_LEVELS > 3 || level != 3);
>  
> -    /* Check there's something for the shadows to map to */
> -    if ( (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt))
> -         || !gfn_valid(d, target_gfn) )
> +    if ( !gfn_valid(d, target_gfn) )
>      {
>          *sp = shadow_l1e_empty();
>          goto done;
> @@ -503,6 +501,13 @@ _sh_propagate(struct vcpu *v,
>          goto done;
>      }
>  
> +    /* Check there's something for the shadows to map to */
> +    if ( !p2m_is_any_ram(p2mt) && p2mt != p2m_mmio_direct )
> +    {
> +        *sp = shadow_l1e_empty();
> +        goto done;
> +    }
> +
>      // Must have a valid target_mfn unless this is a prefetch or an l1
>      // pointing at MMIO space.  In the case of a prefetch, an invalid
>      // mfn means that we can not usefully shadow anything, and so we
> @@ -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?

Thanks, Roger.