RE: [PATCH v3 5/5] intel_iommu: Optimize out some unnecessary UNMAP calls

[Duan, Zhenzhong]

>-----Original Message-----
>From: Peter Xu <peterx@redhat.com>
>Sent: Friday, June 9, 2023 4:34 AM
>To: Duan, Zhenzhong <zhenzhong.duan@intel.com>
>Cc: qemu-devel@nongnu.org; mst@redhat.com; jasowang@redhat.com;
>pbonzini@redhat.com; richard.henderson@linaro.org; eduardo@habkost.net;
>marcel.apfelbaum@gmail.com; alex.williamson@redhat.com;
>clg@redhat.com; david@redhat.com; philmd@linaro.org;
>kwankhede@nvidia.com; cjia@nvidia.com; Liu, Yi L <yi.l.liu@intel.com>; Peng,
>Chao P <chao.p.peng@intel.com>
>Subject: Re: [PATCH v3 5/5] intel_iommu: Optimize out some unnecessary
>UNMAP calls
>
>On Thu, Jun 08, 2023 at 05:52:31PM +0800, Zhenzhong Duan wrote:
>>      while (remain >= VTD_PAGE_SIZE) {
>> -        IOMMUTLBEvent event;
>>          uint64_t mask = dma_aligned_pow2_mask(start, end, s->aw_bits);
>>          uint64_t size = mask + 1;
>>
>>          assert(size);
>>
>> -        event.type = IOMMU_NOTIFIER_UNMAP;
>> -        event.entry.iova = start;
>> -        event.entry.addr_mask = mask;
>> -        event.entry.target_as = &address_space_memory;
>> -        event.entry.perm = IOMMU_NONE;
>> -        /* This field is meaningless for unmap */
>> -        event.entry.translated_addr = 0;
>> -
>> -        memory_region_notify_iommu_one(n, &event);
>> +        map.iova = start;
>> +        map.size = mask;
>> +        if (iova_tree_find(as->iova_tree, &map)) {
>> +            event.entry.iova = start;
>> +            event.entry.addr_mask = mask;
>> +            memory_region_notify_iommu_one(n, &event);
>> +        }
>
>Ah one more thing: I think this path can also be triggered by notifiers without
>MAP event registered, whose iova tree will always be empty.  So we may only
>do this for MAP, then I'm not sure whether it'll be worthwhile..

Hmm, yes, my change will lead to vhost missed to receive some invalidation 
request in device-tlb disabled case as iova tree is empty. Thanks for point out.

Let me collect time diff spent in unmap AS for you to decide if it still worth 
or not.

Thanks
Zhenzhong