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

[Tianyu Lan <ltykernel@xxxxxxxxx>]

On 12/5/2021 6:31 PM, Juergen Gross wrote:
> On 05.12.21 09:48, Tianyu Lan wrote:
> > On 12/5/2021 4:34 PM, Juergen Gross wrote:
> > > On 05.12.21 09:18, Tianyu Lan wrote:
> > > > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> > > >
> > > > 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.
> And does this even work without CONFIG_SWIOTLB_XEN, i.e. without
> pci_xen_swiotlb_detect() being in the system?
Hi Juergen:
	Thanks for your review. This is a issue and I just sent out a v5 which 
decouples the dependency between xen and hyperv.