Re: meaning and use of IOMMU_FLUSHF_added

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 18/08/2021 13:09, Jan Beulich wrote:
> On 18.08.2021 12:51, Jan Beulich wrote:
> > Paul,
> >
> > back at the time I did already question your intended meaning of
> > this flag. I notice that there's presently no consumer of it being
> > set (apart from yielding non-zero flush_flags). I'm afraid this
> > model makes accumulation of flush flags not work properly: With
> > both flags set and more than a single page altered, it is
> > impossible to tell apart whether two present PTEs were altered, or
> > a non-present and a present one.
> >
> > VT-d's flushing needs to know the distinction; it may in fact be
> > necessary to issue two flushes (or a single "heavier" one) when
> > both non-present and present entries got transitioned to present
> > in one go.
> No two (or "heavier") flush looks to be needed upon further reading.
> I did derive this from our setting of "did" to zero in that case,
> but that looks to be wrong in the first place - it's correct only
> for context cache entry flushes. I'll make a(nother) patch ...
Yes, the intention of the flag was simply to allow a 'lighter' flush in 
the case there are no modified entries as part of the accumulation. If 
it is impossible to tell the difference then I guess it's not useful.
  Paul