Re: [Xen-devel] [PATCH] x86/ept: pass correct level to p2m_entry_modify

[Jan Beulich <JBeulich@xxxxxxxx>]

On 03.07.2019 11:43, Roger Pau Monne wrote:
> EPT differs from NPT and shadow when translating page orders to levels
> in the physmap page tables. EPT page tables level for order 0 pages is
> 0, while NPT and shadow instead use 1, ie: EPT page tables levels
> starts at 0 while NPT and shadow starts at 1.
> 
> Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
> one to the level, in order to match NPT and shadow usage.
> 
> While there also fix p2m_entry_modify BUG condition to trigger when
> foreign or ioreq entries with level different than 0 are attempted.
> That should allow to catch future errors related to the level
> parameter.
> 
> Fixes: c7a4c0 ('x86/mm: split p2m ioreq server pages special handling into 
> helper')

A 6-digit hash is definitely too short in the long run. I understand
that this then wants backporting to the 4.12 tree.

> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -946,7 +946,7 @@ static inline int p2m_entry_modify(struct p2m_domain 
> *p2m, p2m_type_t nt,
>                                      p2m_type_t ot, mfn_t nfn, mfn_t ofn,
>                                      unsigned int level)
>   {
> -    BUG_ON(level > 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));
> +    BUG_ON(level != 1 && (nt == p2m_ioreq_server || nt == p2m_map_foreign));

Wouldn't you better leave this alone and add BUG_ON(!level)?

Jan
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