Re: [PATCH v6 00/16] Add Secure TSC support for SNP guests

Peter Gonda
2023-12-06T17:46:56Z

On Tue, Nov 28, 2023 at 6:00 AM Nikunj A Dadhania <nikunj@amd.com> wrote:
>
> Secure TSC allows guests to securely use RDTSC/RDTSCP instructions as the
> parameters being used cannot be changed by hypervisor once the guest is
> launched. More details in the AMD64 APM Vol 2, Section "Secure TSC".
>
> During the boot-up of the secondary cpus, SecureTSC enabled guests need to
> query TSC info from AMD Security Processor. This communication channel is
> encrypted between the AMD Security Processor and the guest, the hypervisor
> is just the conduit to deliver the guest messages to the AMD Security
> Processor. Each message is protected with an AEAD (AES-256 GCM). See "SEV
> Secure Nested Paging Firmware ABI Specification" document (currently at
> https://www.amd.com/system/files/TechDocs/56860.pdf) section "TSC Info"
>
> Use a minimal GCM library to encrypt/decrypt SNP Guest messages to
> communicate with the AMD Security Processor which is available at early
> boot.
>
> SEV-guest driver has the implementation for guest and AMD Security
> Processor communication. As the TSC_INFO needs to be initialized during
> early boot before smp cpus are started, move most of the sev-guest driver
> code to kernel/sev.c and provide well defined APIs to the sev-guest driver
> to use the interface to avoid code-duplication.
>
> Patches:
> 01-08: Preparation and movement of sev-guest driver code
> 09-16: SecureTSC enablement patches.
>
> Testing SecureTSC
> -----------------
>
> SecureTSC hypervisor patches based on top of SEV-SNP Guest MEMFD series:
> https://github.com/nikunjad/linux/tree/snp-host-latest-securetsc_v5
>
> QEMU changes:
> https://github.com/nikunjad/qemu/tree/snp_securetsc_v5
>
> QEMU commandline SEV-SNP-UPM with SecureTSC:
>
> qemu-system-x86_64 -cpu EPYC-Milan-v2,+secure-tsc,+invtsc -smp 4 \
> -object memory-backend-memfd-private,id=ram1,size=1G,share=true \
> -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,secure-tsc=on \
> -machine q35,confidential-guest-support=sev0,memory-backend=ram1,kvm-type=snp \
> ...

Thanks Nikunj!

I was able to modify my SNP host kernel to support SecureTSC based off
of your `snp-host-latest-securetsc_v5` and use that to test this
series. Seemed to work as intended in the happy path but I didn't
spend much time trying any corner cases. Also checked the series
continues to work without SecureTSC enabled for the V.

Tested-by: Peter Gonda <pgonda@google.com>