Re: [PATCH v5 12/15] VT-d: replace all-contiguous page tables by superpage mappings

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

On Fri, May 27, 2022 at 01:19:55PM +0200, Jan Beulich wrote:
> When a page table ends up with all contiguous entries (including all
> identical attributes), it can be replaced by a superpage entry at the
> next higher level. The page table itself can then be scheduled for
> freeing.
> 
> The adjustment to LEVEL_MASK is merely to avoid leaving a latent trap
> for whenever we (and obviously hardware) start supporting 512G mappings.
> 
> Note that cache sync-ing is likely more strict than necessary. This is
> both to be on the safe side as well as to maintain the pattern of all
> updates of (potentially) live tables being accompanied by a flush (if so
> needed).
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

> ---
> Unlike the freeing of all-empty page tables, this causes quite a bit of
> back and forth for PV domains, due to their mapping/unmapping of pages
> when they get converted to/from being page tables. It may therefore be
> worth considering to delay re-coalescing a little, to avoid doing so
> when the superpage would otherwise get split again pretty soon. But I
> think this would better be the subject of a separate change anyway.
> 
> Of course this could also be helped by more "aware" kernel side
> behavior: They could avoid immediately mapping freed page tables
> writable again, in anticipation of re-using that same page for another
> page table elsewhere.

Could we provide an option to select whether to use super-pages for
IOMMU, so that PV domains could keep the previous behavior?

Thanks, Roger.