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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.05.2025 13:40, Roger Pau Monné wrote:
> 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.

In which case why don't we do it in two steps: This patch, and then one
removing the call altogether. Just in case the latter one turns out wrong
for whatever reason.

Jan