[Patch v2 0/4] Introduce Enhanced SMT Protection for SEV-SNP

["Pratik R. Sampat" <prsampat@amd.com>]

Enhanced SMT Protection (ESMTP) allows an SEV-SNP VM to require that,
while one of its vCPUs is in guest mode, every SMT sibling thread on
that physical core is either idle in host mode or running a vCPU the
guest itself has declared a legal sibling.  This mitigates the
side-channel risk of sharing core resources with untrusted host threads
or with another guest.

Unlike core scheduling, where co-residency is a host kernel policy
expressed with cookies, ESMTP is enforced by hardware. The sibling mask
lives in the VMSA.  The host is not trusted to run arbitrary kernel,
userspace, or interrupt-handling work on a sibling thread while an ESMTP
vCPU is active on that core.

Both KVM and the guest fully set the VCPU_SIBLING_MASK, which places
every vCPU of the guest in one group so that any two of them may be
co-resident.  Combined with the ASID check, the sibling of a vCPU in
guest mode is then always either another vCPU of that same guest or a
thread idle in host mode.

Usage
-----
Requires patched OVMF [1] and QEMU [2] builds (unchanged from v1).
Launch an SEV-SNP guest with ESMTP enabled on the sev-snp-guest object:

  -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,esmtp=on

ESMTP is opt-in because it carries a performance cost as VMRUN stalls
until the sibling runs work from a trusted vCPU or is in host idle.

The guest reports the feature in dmesg among the SNP feature names:

  # dmesg | grep -i SEV
  ... SEV-SNP ... ESMTProt ...

Empirical test
--------------
* Identify siblings via:
  cat /sys/devices/system/cpu/cpuX/topology/thread_siblings_list
* Pin the guest on the siblings
* Spawn a load in the guest e.g. via stress-ng --cpu 2 for SMT=2.
  Expect the guest utilization % for both sibling CPUs to be at 100% as
  the vCPU siblings are deemed trusted
* Spawn a load in the host latched onto one of the siblings.
  E.g. taskset -C X stress-ng --cpu 1
* Expect CPU X's utilization to be shared between host and guest %.
  Also expect drop in CPU utilization on the thread Sibling CPU as when
  the host task runs the guest will be forced idle.

Patches based on cryptodev-2.6

v2:
* Move idle wakeup ICR programming to svm_enable_virtualization_cpu()
  instead of sev_hardware_setup() - Sashiko
* Remove cond_sched() in ESMTP exit paths - Sashiko
* Add a VMSA builing of ESMTP fields for hyperv - Sashiko
* Add ESMTP to SNP_FEATURES_IMPL and SNP_FEATURES_IMPL_REQ - Local
  Claude Sashiko instance
* Cover hotplug CPUs / SMT changes by programming the ICR for all CPUs
  - local Claude Sashiko instance
* Add esmtp timeout documentation to kernel paramters

Not syncing msr-index changes since several bits apart from ESMTP are
also changed and perf tooling maintainers generally sync that. However,
if needed I drop in a patch that syncs the MSRs too.

v1: https://lore.kernel.org/kvm/cover.1789399214.git.prsampat@amd.com

[1]: https://github.com/tianocore/edk2/pull/13128
[2]: https://lore.kernel.org/qemu-devel/cover.1789399242.git.prsampat@amd.com/

Pratik R. Sampat (4):
  KVM: SVM: Re-queue events that were never injected
  KVM: SVM: Add host support for Enhanced SMT Protection
  x86/sev: Add guest support for Enhanced SMT Protection
  x86/hyperv: Add guest support for Enhanced SMT Protection

 .../admin-guide/kernel-parameters.txt         | 19 ++++++++
 arch/x86/boot/compressed/sev.c                |  6 ++-
 arch/x86/coco/sev/core.c                      | 13 +++++
 arch/x86/hyperv/ivm.c                         | 11 +++++
 arch/x86/include/asm/cpufeatures.h            |  1 +
 arch/x86/include/asm/msr-index.h              |  5 +-
 arch/x86/include/asm/svm.h                    | 12 ++++-
 arch/x86/include/uapi/asm/svm.h               |  9 +++-
 arch/x86/kernel/cpu/scattered.c               |  1 +
 arch/x86/kvm/svm/sev.c                        | 48 ++++++++++++++++++-
 arch/x86/kvm/svm/svm.c                        | 41 ++++++++++++++--
 arch/x86/kvm/svm/svm.h                        |  2 +
 12 files changed, 157 insertions(+), 11 deletions(-)

-- 
2.43.0