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

From: Tianyu Lan <[email-masked]>
Date: Thu, 3 Jun 2021 23:37:06 +0800

On 6/3/2021 12:02 AM, Boris Ostrovsky wrote:
> 
> 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.

Yes, the dependency is between hyperv_swiotlb_detect() and
pci_swiotlb_detect_override()/pci_swiotlb_detect_4gb(). Now
pci_swiotlb_detect_override() and pci_swiotlb_detect_4gb() depends on
pci_xen_swiotlb_detect(). To keep dependency between
hyperv_swiotlb_detect() and pci_swiotlb_detect_override/4gb(), make 
pci_xen_swiotlb_detect() depends on hyperv_swiotlb_detect() and just to
keep order in the IOMMU table. Current iommu_table_entry only has one
depend callback and this is why I put xen depends on hyperv detect function.

Thanks.