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SubjectRe: [PATCH V7 1/5] swiotlb: Add swiotlb bounce buffer remap function for HV IVM
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On 12/14/21 12:40 PM, Dave Hansen wrote:
> On 12/13/21 8:36 PM, Tianyu Lan wrote:
>> On 12/14/2021 12:45 AM, Dave Hansen wrote:
>>> On 12/12/21 11:14 PM, Tianyu Lan wrote:
>>>> In Isolation VM with AMD SEV, 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.
>>>
>>> This seems to be independently reintroducing some of the SEV
>>> infrastructure.  Is it really OK that this doesn't interact at all with
>>> any existing SEV code?
>>>
>>> For instance, do we need a new 'swiotlb_unencrypted_base', or should
>>> this just be using sme_me_mask somehow?
>>
>>        Thanks for your review. Hyper-V provides a para-virtualized
>> confidential computing solution based on the AMD SEV function and not
>> expose sev&sme capabilities to guest. So sme_me_mask is unset in the
>> Hyper-V Isolation VM. swiotlb_unencrypted_base is more general solution
>> to handle such case of different address space for encrypted and
>> decrypted memory and other platform also may reuse it.
>
> I don't really understand how this can be more general any *not* get
> utilized by the existing SEV support.

The Virtual Top-of-Memory (VTOM) support is an SEV-SNP feature that is
meant to be used with a (relatively) un-enlightened guest. The idea is
that the C-bit in the guest page tables must be 0 for all accesses. It is
only the physical address relative to VTOM that determines if the access
is encrypted or not. So setting sme_me_mask will actually cause issues
when running with this feature. Since all DMA for an SEV-SNP guest must
still be to shared (unencrypted) memory, some enlightenment is needed. In
this case, memory mapped above VTOM will provide that via the SWIOTLB
update. For SEV-SNP guests running with VTOM, they are likely to also be
running with the Reflect #VC feature, allowing a "paravisor" to handle any
#VCs generated by the guest.

See sections 15.36.8 "Virtual Top-of-Memory" and 15.36.9 "Reflect #VC" in
volume 2 of the AMD APM [1].

I'm not sure if that will answer your question or generate more :)

Thanks,
Tom

[1] https://www.amd.com/system/files/TechDocs/24593.pdf

>

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