# Re: \[RFC PATCH V3 09/11\] HV/IOMMU: Enable swiotlb bounce buffer for Isolation VM

- From: Tianyu Lan \<hidden\>
- Date: 2021-06-02 15:01:55 
- List: linux-hyperv
- Page: https://ratatoskr.run/linux-hyperv/2021/06/1400060
- Thread: https://ratatoskr.run/xen-devel/2021/05/1396014/t.md

```text
Hi Boris:
	Thanks for your review.

On 6/2/2021 9:16 AM, Boris Ostrovsky wrote:
>
> On 5/30/21 11:06 AM, Tianyu Lan wrote:
>> @@ -91,6 +92,6 @@ int pci_xen_swiotlb_init_late(void)
>>   EXPORT_SYMBOL_GPL(pci_xen_swiotlb_init_late);
>>   
>>   IOMMU_INIT_FINISH(2,
>> -		  NULL,
>> +		  hyperv_swiotlb_detect,
>>   		  pci_xen_swiotlb_init,
>>   		  NULL);
>
>
> Could you explain this change?

Hyper-V allocates its own swiotlb bounce buffer and the default
swiotlb buffer should not be allocated. swiotlb_init() in pci_swiotlb_init() is to allocate default swiotlb buffer.
To achieve this, put hyperv_swiotlb_detect() as the first entry in the iommu_table_entry list. The detect loop in the pci_iommu_alloc() will exit once hyperv_swiotlb_detect() is called in Hyper-V VM and other iommu_table_entry callback will not be called.
```