Re: [PATCH] x86/p2m-pt: fix p2m_flags_to_access()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/09/2021 08:01, Jan Beulich wrote:
> The initial if() was inverted, invalidating all output from this
> function. Which in turn means the mirroring of P2M mappings into the
> IOMMU didn't always work as intended: Mappings may have got updated when
> there was no need to. There would not have been too few (un)mappings;
> what saves us is that alongside the flags comparison MFNs also get
> compared, with non-present entries always having an MFN of 0 or
> INVALID_MFN while present entries always have MFNs different from these
> two (0 in the table also meant to cover INVALID_MFN):
>
> OLD                                   NEW
> P W   access  MFN                     P W     access  MFN
> 0 0   r       0                       0 0     n       0
> 0 1   rw      0                       0 1     n       0
> 1 0   n       non-0                   1 0     r       non-0
> 1 1   n       non-0                   1 1     rw      non-0
>
> present <-> non-present transitions are fine because the MFNs differ.
> present -> present transitions as well as non-present -> non-present
> ones are potentially causing too many map/unmap operations, but never
> too few, because in that case old (bogus) and new access differ.
>
> Fixes: d1bb6c97c31e ("IOMMU: also pass p2m_access_t to p2m_get_iommu_flags())
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

>
> --- a/xen/arch/x86/mm/p2m-pt.c
> +++ b/xen/arch/x86/mm/p2m-pt.c
> @@ -548,7 +548,7 @@ int p2m_pt_handle_deferred_changes(uint6
>  /* Reconstruct a fake p2m_access_t from stored PTE flags. */
>  static p2m_access_t p2m_flags_to_access(unsigned int flags)
>  {
> -    if ( flags & _PAGE_PRESENT )
> +    if ( !(flags & _PAGE_PRESENT) )
>          return p2m_access_n;
>  
>      /* No need to look at _PAGE_NX for now. */
>