Re: [PATCH v14 03/13] x86/sev: Add Secure TSC support for SNP guests

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/28/2024 1:34 PM, Nikunj A Dadhania wrote:
> Add support for Secure TSC in SNP-enabled guests. Secure TSC allows guests
> to securely use RDTSC/RDTSCP instructions, ensuring that the parameters
> used cannot be altered by the hypervisor once the guest is launched.
> 
> Secure TSC-enabled guests need to query TSC information from the AMD
> Security Processor. This communication channel is encrypted between the AMD
> Security Processor and the guest, with the hypervisor acting merely as a
> conduit to deliver the guest messages to the AMD Security Processor. Each
> message is protected with AEAD (AES-256 GCM). Use a minimal AES GCM library
> to encrypt and decrypt SNP guest messages for communication with the PSP.
> 
> Use mem_encrypt_init() to fetch SNP TSC information from the AMD Security
> Processor and initialize snp_tsc_scale and snp_tsc_offset.

Why do it inside mem_encrypt_init()?

It's better to introduce a snp_guest_init/setup() like tdx_early_init() 
to do all the SNP related setup stuff instead of scattering them all 
around the kernel code.

> During secondary
> CPU initialization, set the VMSA fields GUEST_TSC_SCALE (offset 2F0h) and
> GUEST_TSC_OFFSET (offset 2F8h) with snp_tsc_scale and snp_tsc_offset,
> respectively.
>