Re: [PATCH RFC v9 07/51] x86/sev: Add the host SEV-SNP initialization support

[Tom Lendacky <thomas.lendacky@amd.com>]

On 6/21/23 04:42, Borislav Petkov wrote:
> On Sun, Jun 11, 2023 at 11:25:15PM -0500, Michael Roth wrote:
>> From: Brijesh Singh <brijesh.singh@amd.com>
>>
>> The memory integrity guarantees of SEV-SNP are enforced through a new
>> structure called the Reverse Map Table (RMP). The RMP is a single data
>> structure shared across the system that contains one entry for every 4K
>> page of DRAM that may be used by SEV-SNP VMs. APM2 section 15.36 details
> 
> Rather say 'APM v2, section "Secure Nested Paging (SEV-SNP)"' because
> the numbering is more likely to change than the name in the future. With
> the name, people can find it faster.
> 
>> a number of steps needed to detect/enable SEV-SNP and RMP table support
>> on the host:
>>
>>   - Detect SEV-SNP support based on CPUID bit
>>   - Initialize the RMP table memory reported by the RMP base/end MSR
>>     registers and configure IOMMU to be compatible with RMP access
>>     restrictions
>>   - Set the MtrrFixDramModEn bit in SYSCFG MSR
>>   - Set the SecureNestedPagingEn and VMPLEn bits in the SYSCFG MSR
>>   - Configure IOMMU
>>
>> RMP table entry format is non-architectural and it can vary by
>> processor. It is defined by the PPR. Restrict SNP support to CPU
>> models/families which are compatible with the current RMP table entry
>> format to guard against any undefined behavior when running on other
>> system types. Future models/support will handle this through an
>> architectural mechanism to allow for broader compatibility.
> 
> I'm guessing this is all for live migration between SNP hosts. If so,
> then there will have to be a guest API to handle the differences.
> 
>> SNP host code depends on CONFIG_KVM_AMD_SEV config flag, which may be
>> enabled even when CONFIG_AMD_MEM_ENCRYPT isn't set, so update the
>> SNP-specific IOMMU helpers used here to rely on CONFIG_KVM_AMD_SEV
>> instead of CONFIG_AMD_MEM_ENCRYPT.
> 
> Does that mean that even on CONFIG_AMD_MEM_ENCRYPT=n kernels, host SNP
> can function?

Yes, because CONFIG_AMD_MEM_ENCRYPT is mainly for dealing with the 
encryption bit.

> 
> Do we even want that?

We support that today with SEV and SEV-ES guests. The host/hypervisor 
kernel does not need CONFIG_AMD_MEM_ENCRYPT=y in order to run SEV guests.

> 
> I'd expect that a host SNP kernel should have SME enabled too even
> though it is not absolutely necessary.

I recommend using TSME over SME.

Thanks,
Tom

> 
>> Co-developed-by: Ashish Kalra <ashish.kalra@amd.com>
>> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
>> Co-developed-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>> [mdr: rework commit message to be clearer about what patch does, squash
>>        in early_rmptable_check() handling from Tom]
>> Signed-off-by: Michael Roth <michael.roth@amd.com>
>> ---
>>   arch/x86/coco/Makefile                   |   1 +
>>   arch/x86/coco/sev/Makefile               |   3 +
>>   arch/x86/coco/sev/host.c                 | 212 +++++++++++++++++++++++
>>   arch/x86/include/asm/disabled-features.h |   8 +-
>>   arch/x86/include/asm/msr-index.h         |  11 +-
>>   arch/x86/include/asm/sev.h               |   2 +
>>   arch/x86/kernel/cpu/amd.c                |  19 ++
>>   drivers/iommu/amd/init.c                 |   2 +-
>>   include/linux/amd-iommu.h                |   2 +-
>>   9 files changed, 256 insertions(+), 4 deletions(-)
>>   create mode 100644 arch/x86/coco/sev/Makefile
>>   create mode 100644 arch/x86/coco/sev/host.c
> 
> Ignored review comments here:
> 
> https://lore.kernel.org/r/Y9ubi0i4Z750gdMm@zn.tnic
> 
> Ignoring this one for now too.
>