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

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)
-          NULL,
+          hyperv_swiotlb_detect,

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.




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