Re: [PATCH] IOMMU: avoid double flushing in shared page table case

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.10.2020 17:00, Julien Grall wrote:
> On 20/10/2020 14:52, Jan Beulich wrote:
>> While the flush coalescing optimization has been helping the non-shared
>> case, it has actually lead to double flushes in the shared case (which
>> ought to be the more common one nowadays at least): Once from
>> *_set_entry() and a second time up the call tree from wherever the
>> overriding flag gets played with. In alignment with XSA-346 suppress
>> flushing in this case.
>>
>> Similarly avoid excessive setting of IOMMU_FLUSHF_added on the batched
>> flushes: "idx" hasn't been added a new mapping for.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> TBD: The Arm part really is just for completeness (and hence could also
>>       be dropped) - the affected mapping spaces aren't currently
>>       supported there.
> 
> As I may I have pointed out in the past, there are many ways to screw 
> things up when using iommu_dont_flush_iotlb.
> 
> So I would rather not introduce any usage on Arm until we see a use-case.

"Usage" to me would mean a path actually setting the flag.
What I'm adding here, basically as a precautionary measure,
is a check of the flag. Does your use of "usage" imply you
don't want that either? Just to be sure; I'm okay to drop
the Arm part.

Jan