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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 6/2/21 11:01 AM, Tianyu Lan wrote:
> 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.



Right. But pci_xen_swiotlb_detect() will only do something for Xen PV guests, and those guests don't run on hyperV. It's either xen_pv_domain() (i.e. hypervisor_is_type(X86_HYPER_XEN_PV)) or hypervisor_is_type(X86_HYPER_MS_HYPERV) but never both. So I don't think there needs to be a dependency between the two callbacks.



-boris