Re: [PATCH V4 3/5] hyperv/IOMMU: Enable swiotlb bounce buffer for Isolation VM

[Tianyu Lan <ltykernel@gmail.com>]

On 12/5/2021 4:34 PM, Juergen Gross wrote:
> On 05.12.21 09:18, Tianyu Lan wrote:
>> From: Tianyu Lan <Tianyu.Lan@microsoft.com>
>>
>> hyperv Isolation VM requires bounce buffer support to copy
>> data from/to encrypted memory and so enable swiotlb force
>> mode to use swiotlb bounce buffer for DMA transaction.
>>
>> In Isolation VM with AMD SEV, the bounce buffer needs to be
>> accessed via extra address space which is above shared_gpa_boundary
>> (E.G 39 bit address line) reported by Hyper-V CPUID ISOLATION_CONFIG.
>> The access physical address will be original physical address +
>> shared_gpa_boundary. The shared_gpa_boundary in the AMD SEV SNP
>> spec is called virtual top of memory(vTOM). Memory addresses below
>> vTOM are automatically treated as private while memory above
>> vTOM is treated as shared.
>>
>> Hyper-V initalizes swiotlb bounce buffer and default swiotlb
>> needs to be disabled. pci_swiotlb_detect_override() and
>> pci_swiotlb_detect_4gb() enable the default one. To override
>> the setting, hyperv_swiotlb_detect() needs to run before
>> these detect functions which depends on the pci_xen_swiotlb_
>> init(). Make pci_xen_swiotlb_init() depends on the hyperv_swiotlb
>> _detect() to keep the order.
> 
> Why? Does Hyper-V plan to support Xen PV guests? If not, I don't see
> the need for adding this change.
> 

This is to keep detect function calling order that Hyper-V detect 
callback needs to call before pci_swiotlb_detect_override() and 
pci_swiotlb_detect_4gb(). This is the same for why
pci_swiotlb_detect_override() needs to depend on the 
pci_xen_swiotlb_detect(). Hyper-V also has such request and so make xen 
detect callback depends on Hyper-V one.