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

["Roger Pau Monné" <roger.pau@citrix.com>]

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@suse.com>
> ---
> 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?

Thanks, Roger.