Re: [PATCH v7 0/6] Introduce CET supervisor state support

[Uros Bizjak]

On Mon, May 12, 2025 at 10:57 AM Chao Gao <chao.gao@xxxxxxxxx> wrote:
>
> Dear maintainers and reviewers,
>
> I kindly request your consideration for merging this series. Most of
> patches have received Reviewed-by/Acked-by tags.
>
> Thanks Chang, Rick, Xin, Sean and Dave for their help with this series.
>
> == Changelog ==
> v6->v7:
>  - Collect reviews from Rick
>  - Tweak __fpstate_reset() to handle guest fpstate rather than adding a
>    guest-specific reset function (Sean & Dave)
>  - Fold xfd initialization into __fpstate_reset() (Sean)
>  - v6: https://lore.kernel.org/all/20250506093740.2864458-1-chao.gao@xxxxxxxxx/
>
> == 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 kernel disables 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].

Dear Chao,

I wonder if the same approach can be used to optimize switching of
Intel PT configuration context. There was a patch series [1] posted
some time ago that showed substantial reduction of overhead when
switching Intel PT configuration context on VM-Entry/Exit using
XSAVES/XRSTORS instructions:

Manual save(rdmsr):     ~334  cycles
Manual restore(wrmsr):  ~1668 cycles

XSAVES insturction:     ~124  cycles
XRSTORS instruction:    ~378  cycles

[1] https://lore.kernel.org/lkml/1557995114-21629-1-git-send-email-luwei.kang@xxxxxxxxx/

Best regards,
Uros.