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Re: meaning and use of IOMMU_FLUSHF_added
- To: Paul Durrant <paul@xxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 18 Aug 2021 14:09:41 +0200
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- Delivery-date: Wed, 18 Aug 2021 12:09:54 +0000
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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 ...
Jan
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