[PATCH v6 0/7] Introduce CET supervisor state support

Chao Gao
2025-05-06T09:32:37Z

Dear maintainers and reviewers,

I kindly request your consideration for merging this series. The changes
between v5 (posted one month ago) and v6 are minimal, and most of the
patches have received Reviewed-by/Acked-by tags.

Thanks Chang, Rick, Xin and Sean for their help with this series.

== Changelog ==
v5->v6:
- Collect reviews from Chang and Rick
- Reset guest default size to FPU legacy size when XSAVE is not available
or not enabled (Chang)
- Drop guest default user features and size, as they will not differ from
host FPUs (Rick)
- v5: https://lore.kernel.org/kvm/20250410072605.2358393-1-chao.gao@intel.com/

== Background ==

CET defines two register states: CET user, which includes user-mode control
registers, and CET supervisor, which consists of shadow-stack pointers for
privilege levels 0-2.

Current kernels disable shadow stacks in kernel mode, making the CET
supervisor state unused and eliminating the need for context switching.

== Problem ==

To virtualize CET for guests, KVM must accurately emulate hardware
behavior. A key challenge arises because there is no CPUID flag to indicate
that shadow stack is supported only in user mode. Therefore, KVM cannot
assume guests will not enable shadow stacks in kernel mode and must
preserve the CET supervisor state of vCPUs.

== Solution ==

An initial proposal to manually save and restore CET supervisor states
using raw RDMSR/WRMSR in KVM was rejected due to performance concerns and
its impact on KVM's ABI. Instead, leveraging the kernel's FPU
infrastructure for context switching was favored [1].

The main question then became whether to enable the CET supervisor state
globally for all processes or restrict it to vCPU processes. This decision
involves a trade-off between a 24-byte XSTATE buffer waste for all non-vCPU
processes and approximately 100 lines of code complexity in the kernel [2].
The agreed approach is to first try this optimal solution [3], i.e.,
restricting the CET supervisor state to guest FPUs only and eliminating
unnecessary space waste.

Key changes in this series are:

1) Fix existing issue regarding enabling guest supervisor states support.
2) Add default features and size for guest FPUs.
3) Add infrastructure to support guest-only features.
4) Add CET supervisor state as the first guest-only feature.

With the series in place, guest FPUs have xstate_bv[12] == xcomp_bv[12] == 1
and CET supervisor state is saved/reloaded when xsaves/xrstors executes on
guest FPUs. non-guest FPUs have xstate_bv[12] == xcomp_bv[12] == 0, then
CET supervisor state is not saved/restored.

== Performance ==

We measured context-switching performance with the benchmark [4] in following
three cases.

case 1: the baseline. i.e., this series isn't applied
case 2: baseline + this series. CET-S space is allocated for guest fpu only.
case 3: baseline + allocate CET-S space for all tasks. Hardware init
optimization avoids writing out CET-S space on each XSAVES.

The performance differences in the three cases are very small and fall within the
run-to-run variation.

[1]: https://lore.kernel.org/kvm/ZM1jV3UPL0AMpVDI@google.com/
[2]: https://lore.kernel.org/kvm/1c2fd06e-2e97-4724-80ab-8695aa4334e7@intel.com/
[3]: https://lore.kernel.org/kvm/2597a87b-1248-b8ce-ce60-94074bc67ea4@intel.com/
[4]: https://github.com/antonblanchard/will-it-scale/blob/master/tests/context_switch1.c

Chao Gao (4):
x86/fpu: Drop @perm from guest pseudo FPU container
x86/fpu/xstate: Differentiate default features for host and guest FPUs
x86/fpu: Initialize guest FPU permissions from guest defaults
x86/fpu: Initialize guest fpstate and FPU pseudo container from guest
defaults

Sean Christopherson (1):
x86/fpu/xstate: Always preserve non-user xfeatures/flags in
__state_perm

Yang Weijiang (2):
x86/fpu/xstate: Introduce "guest-only" supervisor xfeature set
x86/fpu/xstate: Add CET supervisor xfeature support as a guest-only
feature

arch/x86/include/asm/fpu/types.h | 64 +++++++++++++++++++++++--------
arch/x86/include/asm/fpu/xstate.h | 9 +++--
arch/x86/kernel/fpu/core.c | 46 +++++++++++++++-------
arch/x86/kernel/fpu/init.c | 1 +
arch/x86/kernel/fpu/xstate.c | 58 +++++++++++++++++++++-------
arch/x86/kernel/fpu/xstate.h | 5 +++
6 files changed, 137 insertions(+), 46 deletions(-)

base-commit: 960bc2bcba5987a82530b9756e1f602a894cffa4
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2.47.1