[PATCH v4 0/5] Enable Secure TSC for SEV-SNP

Nikunj A Dadhania
2025-03-10T06:40:04Z

The hypervisor controls TSC value calculations for the guest. A malicious
hypervisor can prevent the guest from progressing. The Secure TSC feature for
SEV-SNP allows guests to securely use the RDTSC and RDTSCP instructions. This
ensures the guest has a consistent view of time and prevents a malicious
hypervisor from manipulating time, such as making it appear to move backward or
advance too quickly. For more details, refer to the "Secure Nested Paging
(SEV-SNP)" section, subsection "Secure TSC" in APM Volume 2.

This patch set is also available at:

https://github.com/AMDESE/linux-kvm/tree/sectsc-host-latest

and is based on kvm-x86/next

Testing Secure TSC
-----------------

Secure TSC guest patches are available as part of v6.14-rc1.

QEMU changes:
https://github.com/nikunjad/qemu/tree/snp-securetsc-latest

QEMU command line SEV-SNP with Secure TSC:

qemu-system-x86_64 -cpu EPYC-Milan-v2 -smp 4 \
-object memory-backend-memfd,id=ram1,size=1G,share=true,prealloc=false,reserve=false \
-object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,secure-tsc=on,stsc-freq=2000000000 \
-machine q35,confidential-guest-support=sev0,memory-backend=ram1 \
...

Changelog:
----------
v4:
* Rebased on top of latest kvm-x86/next
* Collect Reviewed-by from Tom
* Use "KVM: SVM" instead of "crypto: ccp" (Tom)
* Clear the intercept in sev_es_init_vmcb() (Tom)
* Differentiate between guest and host MSR_IA32_TSC writes (Tom)

v3: https://lore.kernel.org/kvm/20250217102237.16434-1-nikunj@amd.com/
* Rebased on top of kvm-x86/next
* Collect Acked-by
* Separate patch to add missing desired_tsc_khz field (Tom)
* Invoke kvm_set_msr_common() for non-SecureTSC guests (Tom)
* To align desired_tsc_khz to 4-byte boundary, move the 2-byte pad0 above it (Tom)
* Update commit logs (Tom, Sean)

Ketan Chaturvedi (1):
KVM: SVM: Enable Secure TSC for SNP guests

Nikunj A Dadhania (4):
x86/cpufeatures: Add SNP Secure TSC
KVM: SVM: Add missing member in SNP_LAUNCH_START command structure
KVM: SVM: Add GUEST_TSC_FREQ MSR for Secure TSC enabled guests
KVM: SVM: Prevent writes to TSC MSR when Secure TSC is enabled

arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/svm.h | 1 +
arch/x86/include/uapi/asm/kvm.h | 3 ++-
arch/x86/kvm/svm/sev.c | 22 ++++++++++++++++++++++
arch/x86/kvm/svm/svm.c | 20 ++++++++++++++++++++
arch/x86/kvm/svm/svm.h | 11 ++++++++++-
include/linux/psp-sev.h | 2 ++
7 files changed, 58 insertions(+), 2 deletions(-)

base-commit: c9ea48bb6ee6b28bbc956c1e8af98044618fed5e
--
2.43.0