Re: [PATCH V7 1/5] swiotlb: Add swiotlb bounce buffer remap function for HV IVM

[Dave Hansen <dave.hansen@intel.com>]

On 12/14/21 2:23 PM, Tom Lendacky wrote:
>> 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].

Thanks, Tom, that's pretty much what I was looking for.

The C-bit normally comes from the page tables.  But, the hardware also
provides an alternative way to effectively get C-bit behavior without
actually setting the bit in the page tables: Virtual Top-of-Memory
(VTOM).  Right?

It sounds like Hyper-V has chosen to use VTOM instead of requiring the
guest to do the C-bit in its page tables.

But, the thing that confuses me is when you said: "it (VTOM) is meant to
be used with a (relatively) un-enlightened guest".  We don't have an
unenlightened guest here.  We have Linux, which is quite enlightened.

Is VTOM being used because there's something that completely rules out
using the C-bit in the page tables?  What's that "something"?