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Re: meaning and use of IOMMU_FLUSHF_added

On 18/08/2021 13:09, Jan Beulich wrote:
On 18.08.2021 12:51, Jan Beulich wrote:

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.




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