Re: [PATCH v2 4/6] VT-d: restrict iommu_flush_all() to cache writeback

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 03, 2023 at 11:46:11AM +0200, Jan Beulich wrote:
> We don't need to invalidate caches here; all we're after is that earlier
> writes have made it to main memory (and aiui even that just in case).
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> This, aiui, being an analogue to uses of iommu_sync_cache() (just not
> range restricted), I wonder whether it shouldn't be conditional upon
> iommu_non_coherent. Then again I'm vaguely under the impression that
> we had been here before, possibly even as far as questioning the need
> for this call altogether.

I think yes, it would better be only done for iommu_non_coherent.  Yet
in that case I wonder why we need this wide flush.  In principle all
accesses should already have their own write-back calls if the IOMMU
is non-coherent?

There's maybe the call from vtd_crash_shutdown() which I guess could
trigger in the middle of some interaction with the IOMMU, but at that
point do we really care to flush anyway if Xen is going to crash?

Otherwise it seems fine to switch to write-back.

Thanks, Roger.