Subject: Re: [PATCH V7 1/5] swiotlb: Add swiotlb bounce buffer remap function
 for HV IVM
To:     Tom Lendacky <thomas.lendacky@amd.com>,
        Tianyu Lan <ltykernel@gmail.com>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com, wei.liu@kernel.org,
        decui@microsoft.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, davem@davemloft.net, kuba@kernel.org,
        jejb@linux.ibm.com, martin.petersen@oracle.com, arnd@arndb.de,
        hch@infradead.org, m.szyprowski@samsung.com, robin.murphy@arm.com,
        Tianyu.Lan@microsoft.com, michael.h.kelley@microsoft.com
Cc:     iommu@lists.linux-foundation.org, linux-arch@vger.kernel.org,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-scsi@vger.kernel.org, netdev@vger.kernel.org,
        vkuznets@redhat.com, brijesh.singh@amd.com, konrad.wilk@oracle.com,
        hch@lst.de, joro@8bytes.org, parri.andrea@gmail.com
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 <198e9243-abca-b23e-0e8e-8581a7329ede@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <7afc23c3-22e7-9bbf-7770-c683bf84a7cc@intel.com>
Date:   Tue, 14 Dec 2021 14:40:36 -0800
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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"?